Prelude...$80 max.
The other two Unis...$40 max.
You'll probably need to buy new chain and rear cogs. Some many need new tires and brake pads. So figure another $60 bucks. Plus $30 to $50 for a tune up if you don't know how to fix the bike. If seller will not lower price, then wait for a better deal.
FYI, I paid $25 for a 1985 Bridgestone 400, and $5 for a 1995 Cannondale R600.