Decide how much you want to make (profit) off each dishcloth (over the cost of the yarn, of course), and there's your price. Then, let your friends know how much the yarn cost and how much over this cost making them is worth to you. It may deter some, but if others really want it, they'll pay.
Or maybe you should first decide whether you really want to spend a lot of time making the same thing over and over and over and over again.
(OR you could do a little drawing and just give out one as a freebie!)