Not always - I don't know about your local library, but here they have a monthly book sale at the main city library. They sell off surplus copies of titles they already have at ridiculously cheap prices - I picked up Asimov's collected Foundation Series for something like $1.50 - hardback, dust jacket and everything.
Sure, some are in horrible shape, but for $0.75 to $2.00 you can't really complain. I've found some remarkable (to me) treasures - old Time-Life publications like "Great Ages of Man" and such that I'd never be able to find at most traditional book stores.
I highly recommend checking it out if your local library has a sale like this. There can be some serious crowds there but in general it's a great way to get some good literature, as well as support your library system.