I've found over the years that if you don't mind rummaging through the shelves of thrift shops, you can sometimes get nice old s-f books cheap.
Earlier this year, I really confirmed this to myself when I picked up four great old Heinlein books at a thrift store in Winnsboro - where I used to live and work - for fifty cents each.
Friday afternoon I was home (because I have to cover football Friday nights, I get out early from the office on Fridays) and I took the opportunity to browse a thrift shop just a block from where I live. I found a copy of Asimov's "The Hugo winners Vol. 1" for 50 cents.
Good deal, thinks I. Then Saturday morning, I went to a book sale held by the Friends of the Library in New Boston, where I work now. This is the same group that hosted my reading of "A Rocket for the Republic" on Oct. 18.
I picked up a copy of "Lucifer's Hammer", "1984", "The Science Fiction Hall of Fame, Volume IIA" and a Ray Bradbury collection, "Quicker than the Eye", for fifty cents each.
BUT, here's the best part. The hardcovers were a buck each, and I found a perfect copy of "For Us, the Living" there. Two years ago, I would have paid $25; now I got it for a buck, a dollar, uno greenback.