Well, I'm not Doc, but I can steer you in the right direction, as that's primarily what my job responsibilities are. 
While lowest operation cost is relatively easy to find, I'm assuming purchase cost also plays into this as well? If so, how much do you expect to be printing per month/year for the next 3-5 years?
If you're only printing 100-pages per month or so, honestly your best bet would be to find the cheapest printer on clearance(like sub-$50), and when it runs out of toner/ink, get it refilled as Doc mentioned - but understand that being a low-end printer, it might not last much past the first cartridge.
The second option would be to try and find something used in the HP Laserjet 4000/4050/4100/4200 families. Some of them are getting pretty old, but as Doc mentioned above, they're damn near indestructible and can be repaired/refurbished for millions of pages of use. I've heard of wholesalers offering rebuilt printers for $200-$250, which is about the cost of the parts to rebuild it!
Any of those HP Laserjets I listed will cost approximately $0.008-$0.015 per-page(not counting paper) to operate, whereas most inkjet printers are in the $0.08-$0.15 per-page range for B&W prints.
A couple things to keep in mind though: Color printing gets very expensive, very fast. The average HP Color Laserjet is in the $0.095-$0.18 per-color page range, while inkjets can cost well over $0.25 per-page!
That may not mean much if you're just printing the occasional document at home, but at 1000-color pages per-month, that's some real money.
And another note - the HP Laserjet 1100/1200 printers, as well as the newer P2035, P4015(and most of the other "P" models) are actually much less efficient than the older models, costing $0.025-$0.045 per-page.
Yeah, I spend way too much time with this data...