Well, it depends, but because of the story arcs, zombie pages (good ones, with multiples) are few and far between. Most were pre 48. I can only think of a few that we've seen post 48 that were stunners.
A page with a good kill from Michonne or Rick on ebay will most likely fetch anywhere from 700-1500 depending on what issue it is, size of art, context, detail, etc.
Anything pre 48 with a lot of zombies can be as high as 2K. There's a 2 page splash on ebay that i think may be one of the best spreads from the book, and that should hit cover type pricing (3-4K). It's a terrifying piece. So awesome.