Yes, most (probably all) external USB or Firewire drives are compatible with Mac. I have one I got off the shelf (well, actually it was one that came with a USB enclosure that I took out the drive and put into a Firewire enclosure; more on that in a sec) sitting right here on my desk next to my Macbook. 250GB and it was about $120.
Caveat: I would strongly recommend that you get a Firewire enclosure. Why? Well, it's faster, but also aside from the limited number of USB ports you might have available so you won't be taking up one with it, I've encountered at least one problem with a USB external DVD burner. It had both USB and Firewire, and when hooked up to my Mac via USB I had problems getting it to work. When I hooked it up with Firewire it performed flawlessly. Might have been because it was my old Mac Mini, but that was my experience, so I recommend that a Firewire external enclosure be used with a Mac.