Donkey Kong is pretty high. I see Donkey Kongs in my area all the time for $550 or less.
Ghost n' Goblins on the other hand is unique hardware and I can't remember seeing one in my area ever. Doesn't mean they're necessarily rare or go for a lot. But if it's in good shape, $300 is pretty fair for most solid cabs. I'd think that wouldn't be a bad buy.
Keep in mind though that the arcades can take up a LOT of space, and they can be addictive to collect for as cool "opportunity" buys. Make sure they're games you really want to keep around before making the plunge.