I don't think you'll find any rule of thumb for this. If you're a big-name artist, you'll get a lot more for your prints than a lesser known artist will.

Unless you already have a following, don't expect to sell a lot of prints. Rosity had a print shop here for awhile, but few people bought anything.

You might check Cafepress or Zazzle. It's a low-risk way to sell prints. And it will give you an idea of what the base cost will be.