Get the best one you can afford, seriously. A truly shitty couch will just make your butt hurt, and you will regret the money you spent very quickly.
If you scope out the OK stores like Jennifer Convertibles or Crate & Barrel, sometimes you can score a floor sample for waaaay cheaper, with only minor dings, for a few hundred bucks. If you move, you can turn around, sell it later and recoup some cash.
Or try Ikea, although they have a delivery charge, the stuff is still way cheaper and pretty good. Got a friend with a vehicle?
Although I lived on a futon couch for a few years, I still maintain that they suck. If you have to do it, go to Economy Foam & Futon on Houston and Avenue A. Get a metal frame one, and the most expensive multilayer futon you can find, seriously.