I've actually sold more coffee mugs than I have poster prints. The quality of the stuff is top-notch, but the drawback is that you only get a percentage of the total of what you sell, and it's really not all that much. If you're looking to get a better paying rate, I'd suggest going to a site like redbubble.com or Zazzle.com. Not knocking Deviantart's printing thing, but I just haven't been that impressed with their prices.
So the royalties suck, eh? Hahahaa, guess that's to be expected. It's only profitable if you sell a lot. I guess it just sounds so easy. You just click a few buttons and dA takes care of the rest. If you don't mind me asking, how much money have you made from selling prints?