Inspired by the most/least purchase price thread...just add up parts prices since labor could be anything from DIY to taking the car to Fall-Line and could wildly skew the numbers. Let's go!
Least: Donor AE92. Never got paperwork for it, took about $2k worth of stuff out of it...as in, buying that stuff even used would have cost more than I paid for the whole car.
Most: Driver AE92, has about $10-$15k of parts in it, but about half of that is import duties + shipping + other import costs.
Least - 98 Ford Explorer - owned it 10 years and other than wear items, I spent $2500 on front end work. Half of that was only because the ABS sensors died and Ford, in their wisdom, made them part of the spindles.
Most - 98 VW Passat - I don't even want to talk about it.
Least, current civc. Nothing more than oil and tires.
Most, about a tie between the 2 neons. Both needed new transmissions. One went through 2 engines. The other needed $1k+ in repairs to pass inspection when I bought it. Both nickle and dimed me with constant issues.
Most: the cutlass. $300, and I haven't even fixed the rust yet.
The least I've put into parts was my '95 Passat VR6. A friend of mine bought this car many years ago. He drove it, loved it, and had many fun Volkswagen owner experiences. He then came into a deal on a manual Fusion and parked this car in his parents driveway. It sat for at least five years, filled with water, sank into the ground, and he offered it to me for $200. I towed it, drained the water, borrowed a battery to put in it, aired up the tires, then drove it a hundred or miles or so before trading it to another friend for a pair of 900s.
The most? Berkeley you for even making me think about it. 
Least: Hard to say... Over 11 years and 329K miles, I put little more than maintenance & wear items into my '03 TDI. Very few actual "repairs" due to things breaking. So far, the Grand Caravan has been pretty good, but I've only had it a year.
Most: Tied between my Cummins and the E30. Neither would run for more than a month before something broke. I don't like to think about the buckets of cash I poured into both those effin' turds in an attempt to keep them running... only to finally give up and sell them for a huge loss.
Least: 2000 Taurus wagon, 1 serpentine belt and 1 front wheel bearing in nine years, 160,000+ miles. Not counting trans fluid changes every 30,000.
Most: Lotus Europa, but the bulk of that was not the car's fault.
$400 MR2 got a few grand in parts and repairs. Ugh.
My $500 Impala wagon got zero money put into it other than gas. An uncle repaired the frame for free, I doubt I ever changed the oil, I may have washed it once.
Least would be under $200 for a 2000 Ford Focus that I put 40,000 miles on, not counting oil, filters, and tires. I just replaced the cabin filter air box to fix an issue with water leaks, the parking brake handle, and upgraded the wiper switch to a variable speed model. I think I changed the spark plugs once, too.
Most? I'm probably about $5,000 or so into parts for my Dodge Dart. Probably more.
Most:
Look no further than my avatar... Bought my 1G AWD Talon for $2800 back in 2003. Not sure on the exact number spent to keep it on the road, but the list would be LONG. Two blown motors, an A/T to M/T swap, a complete suspension overhaul, multiple turbo failures, multiple tire/wheel combo's and other crap later, I've easily sunk over three times what I bought it for into it. Sad part is, the car has maybe gone 35k since I bought it (mostly sitting broken), and has never really been modded deeper than bolt ons...
Honorable mention goes to my ION Red Line, although it's mostly my own doing, and nothing against what was a very reliable car. First, I RELIGIOUSLY changed the oil using AMSOil. Secondly, I changed the radio many times, at first due to the original going bad, then trying different USB/Aux setups so I could use my Ipod. Lastly, I spent gobs of money on different intake/cooling/exhaust/pulley mods and necessary tuning wanting to make it faster. However, I was never able to launch it good enough to break out of the 14's in the 1/4. On the street, it delivered mixed results against commonly found "Stage 2" WRX's and bolt on DSM's, then got absolutely dusted by a friends paper filter stock LS1 Trans Am. The same money into my DSM would've made it run circles around the Saturn.
Least:
Has to be my old DD Geo Prizm. Bought it for $1100. Somehow managed to blow a 4AFE, but got a whole 'nother running/driving Prizm with low miles that had been crashed for $300. One set of tires, a clutch, some new suspension goodies and a few parts sourced at a local U-pull lot probably totaled another $800. Roughly $2200 invested, plus dirt cheap PLPD insurance, and it gave me 3.5 years of service. Plus it got around 30 in the city and sometimes over 40 on the highway. Sold it to a buddy who DD'ed it for another 4 years getting similar results.
Least '89 SHO with a "Bad Trans" Morons popped the shifter cables off the shifter. Free fix, drove it for awhile.
Most, berkeleying '00 S10 4cyl/5sp. Transmission every 35k miles like clockwork, 2 rear ends, multiple other problems, etc Between purchase price and what I dumped into it, probably $16k. 
Least was the wife's 2000 Buick LeSabre. Got 13 years and 180K miles out of it with just routine maintenance. It still had the original battery and ran fine when she decided she wanted a new car.
Most is my 2001 Focus. No problems with car, but things like superchargers, LSD's, disk brake conversions, etc. aren't cheap.
Least: Wife's '99 Pontiac Sunfire. Truly the cockroach of the automotive world. Basic maintenance and one fuel pump.
Most: It's best not to know - ignorance is bliss; I just want to be happy.
Usually I pour so much into a vehicle, that when something goes wrong, I might as well just keep going.
My parents probably have over $5k into their Toyota Echo.
I'm waiting a 2.7TT allroad owner 
Least, I bet that would be my old hotrod 510. I had to replace a cylinder head once. Less than 500 in that, and a couple diy clutch replacements. I drove the wheels off it for ten years. Most? Well, there's this Open class 4wd Mazda stage rallycar in the garage, see, with a frankenstein drivetrain, and I don't eeeeeven wanna know.. Cuz way, way more than it's worth
captdownshift wrote: I'm waiting a 2.7TT allroad owner
Dude, don't hate. And I want another one
Edit: bought '01 Allroad in '08 for $7500. Drove it for two years, put about $4000 into it (timing belt and suspension) and sold it for $7500. Not terrible I don't think.
The least I spent was on my old 98 Lumina. I had to fix the fuel pump which was around $400-$500, replace brake pads, shoes, oil changes, and that's about it.
The most I spent is on my E36 328i. I spent probably $3K on parts fixing it total on a clutch, pressure plate, flywheel, 2 clutch slave cylinders, 2 guibos, shifter bushings, crank position sensor, rear wheel bearings, 2 steering racks, shocks, and a few other things. Sadly, I may need to sell it at a huge loss if a bunch more parts break.
My 924s wins both category's
The problem is when you have a car for more then 20 years and 350K miles the sum of everything I have spent on it will be huge but if you divide that same cost by the millage (getting $$$$/mile) I bet it is the least expensive car I have had.
Least: Besides oil changes and gas, a $3 heater hose on my 1991 Ranger 2.3L that I bought for $600 and drove (beat on haha) for 2 years.
Most: 1999 Mustang Cobra vert. Owned by 2 rich kids that beat it to an inch of it's life before I got it. I owned it for 4 years and and in that time I replaced the clutch twice, the entire rear end twice (diff casing, gears, LSD and one stub axle), the shifter assembly, both front wheel hubs and then it developed a nasty knock, which after spending almost $1k to diagnose and coming up with nothing, led me to get rid of it. That stupid car almost led me to financial ruin.
Least: Ford Focus. I put an ignition coil in it and it has been running great. I guess I did oil/plugs too but that is just general maintenance.
Most: BMW E39 M5. Even doing all the work myself, parts were expensive. I racked up quite a cost list in the 1.5 years I owned it, and I didn't even drive it for about 8 months. I've replaced twice as many parts on my 318ti for less than what I put in my M5, but it is creeping up there. It will have a fully refreshed suspension by the end of this summer though.
My '90 Track Rat Miata I parted out at the beginning of last year.
There was north of $4000 in the suspension and brakes alone.
Least - I bought this LeBaron for $125 including a safety inspection. Drove it for over a year. Had to put in an O2 sensor and a borrow a battery for a bit. Never did an oil change. Drove it on the highway once where it promptly ran dry of oil (oddly never leaked or burnt a drop in town). Poured in some used oil from the oil change on the wife's minivan then drove for another three months. Traded it for a Lada Niva after changing insurance companies.
Most? My wife's Nissan Quest - we had it a long time but it got real expensive at the end.
Forgetting about hot rods and race cars---
Least: Kia Soul; 100k miles on oil changes,1 set of wiper blades and 1 set of Continental DWS.
Most: '03 Dodge Ram 2500; a dropped valve due to a broken valve spring at 150K miles took out a piston and consequently the cylinder wall and entire block. Thousands of $$$ in a split second. Prior to that it had only needed normal wear items.
Jeff
Least.... $25 Vega... it was pretty rusty, but I put 5000 miles on it, and then drove it to the junkyard where they gave me $50
Most... cash? Are we adjusting for inflation?? If we are adjusting for inflation then the most I've spent was restoring the 1970 428 Super Cobra Jet Mach 1, it cost me about $14,000 back in 1990-93. My current ride, Surreptitious, my tweaked Corolla has about $18,000 in her, but that is 2010 $$, not 1990 $$
An online calculator estimates the Mach1 in 2010 $$ would have been about $25,000
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