I don't think there's a reliable price guide. Old D&D books aren't very rare, for the most part, but you have serious price gouging from some sellers. Pretty much any 1st edition AD&D book should be around $10-$20. I remember locally seeing a used book store trying to sell them for $75.
Some of the old adventure modules are genuinely collectible, and like >>220573 said the original Chainmail books, the things I see that are pricier are out of print boxed sets like Paranoia 2nd edition or TSR's Marvel Superheroes Advanced Set.