The Campaign Builder's Guild

Can you give an idea of some generic target numbers/DCs/whatever you call them that you might use? What counts as easy, medium and hard, roughly?

Xath, the general rule of thumb I use for my games in regards to weird equipment like prosthetics is: If you go through the RP to get it, risking damage to yourself in order to try to get it, then it's a character reward, and is essentially free in response to the challenge of obtaining it. Otherwise, to just "purchase" or have the item, either because you obtained the item without facing any danger or the character possesses it before the game starts, requires it  to be counted as a special ability (A boon, in this case). That's how I have always considered it, and for the most part, it's worked out pretty well, so long as the item itself is balanced.

Sure thing.  For skill checks I generally consider about an 8 fairly easy, a 10 pretty challenging, and a 12+ extremely difficult.