He Who Whispers(John Dickson Carr) is a brilliant horror/mystery book by John Dickson Carr and it's at 6.39 right now, I think. If you consider that cheap and like mysteries at all, it's definitely worth a read.
The Thorndyke mysteries are pretty great and cheap too(99 cents for like 4 novels) but you can find them at Gutenberg too, I think.
"I am not a serial killer" is a great book especially if you like shows like Dexter, but it's 9 dollars, so that might be a bit more expensive than you are looking for.