I've been doing a bit of bargain hunting lately for RPG related items and I've had some good luck including old issues of Dragon (they're "reading" copies, not collector's items) and GDW and Judges Guild Traveller materials (none of which cost more than $0.89)
In the foreground you'll see the recently published items - Gygax and The Manor were both full price, and well they should be. Indeed, I'd have paid more for The Manor if necessary.
Monsters of the Endless Dark was less than $1 and looks to have a lot of unusual monsters that I can drop into just about any game. It's d20, but that's easily adapted.
Although I'm happy to have acquired Robins' Laws of Good Game Mastering, let me say, save your money. If you must have it, get the PDF unless you happen to stumble on a reasonably priced print edition. Some of the prices this is listed for are just absurd and I haven't read anything in the content that justifies that.