http://www.horrormovies.com/
That's an awesome horror movie site. Pretty much all the movies are dirt cheap and they have a lot of the rare ones.
I would definitely get all 3 Evil Deads, like Gabe said. Also, get Suspiria, Phenomena, Tenebre, Deep Red, and even Inferno. All of those are Dario Argento movies. If you've never seen an Argento film, you're missing out on some seriously good horror.
There's also movies like the Romero Zombie Quadrilogy: Night of the Living Dead, Dawn of the Dead, Day of the Dead, Land of the Dead. The Hellraiser series was tremendous. Stephen King's It and The Shining. The Exorcist and the Exorcist III. There is a 50 pack of classic horror movies and another 50 pack of Chilling Classics. Pretty much all 100 of those are quality movies especially for the price you'd be getting them (about 40 cents per movie). The 50 Horror Classics are all from before 1960 though.
I have hundreds of horror movies, so if you want more suggestions I can try to help.