Depends on the person selling the tank. I've seen people sell a 55 for 50 dollars or less, but others try to sell it for 70.
I would expect close to 100 dollars or more for a tank bigger than 75 gallons probably. Sometimes I see them for less. Big tanks are hard to sell, so sometimes people will lower their prices to sell them. If everyone is selling them for too much, wait for one of those '1 dollar per gallon' sales at a petstore. Save up that much money. It doesn't take that long :D
I look on craigslist to see what's for sale in my area :)