Oh, not much at all. It's a funny story, but I compressed it; I basically had to repeat the process a couple times with each store, selling each one the other's $1 books. They really don't pay much at all, but since I know a lot about literature, as well as what each store would purchase (and their current inventory, since I spent so much time browsing), I was able to turn $50 into $350 in a couple of visits back and forth.
I've had what I'd consider some great book scores (for me -- a $350 William Burroughs book, some out of print David Foster Wallace first editions, etc.) though I always keep them for my greedy self.
It's getting a lot harder now that they seem to double check everything online. Still, occasionally one slips through.