What are you looking to do? How are you planning to use the bike?
You won't find $200-$300 builds on any bike used for racing. I mean...maybe...but you'd be a severe disadvantage in a number of ways.
Weight
Tires
Shifting
Old cheap bikes prior to the internet all weighed a ton. Occasionally you'll find a higher-end bike for sale, but usually the owner knows its value and wants a few hundred for it. My 1980 Razesa was high-end small numbers hand built in Spain and it still weighs more than most entry level 2012 road bikes. You'll find some relatively lightweight frames from the 80's, but with the fixie/vintage crowd hunting, many unique frames have increased in price, and if you spend $150-200 on a nice frame, you'll still have a heavy bike.That's because of components and tires.
Tires and wheels have changed a fair amount in the last 20 years. Wheels are designed for a certain amount of flex while still being lightweight. Old tires are usually dry-rotted and need replaced. You can buy an older bike, get some new tires, but it may not be as comfortable as a newer bike with new wheels designed for a comfortable ride.
Lastly, component weights have change drastically since the 80's. Carbon forks, seat posts, handelbars, wheels, shifter/brake combos, seats, all have reduce bike weight, but also cost a pretty penny. Downtube shifters are light, but they could also get you killed if you were riding in a tight race. If you were racing you'd want brake mounted shifter combos.
So, if you want a bike to put miles on, any bike with good tires do.
If you want a bike to race, to keep up with the pack, and impress your friends, you'll need to spend a little.
Tri bikes are very specific. They are a relatively new style of bike. Tri bikes from the 90's were just a high-end lightweight road bike, now they are built for aero, not for comfort. Skinny frames and wheels, aero bars, aero seatposts. They are usually made to be stiff and lightweight so you can efficiently put the power down. Carbon is common, and wheels/frame will cost you $300 alone, even used, even some triathletes junk will cost you $300.