I recently had to do this with a new client. I'm printing their monthly letter on my home printer and mailing it out for them. They sent me envelopes and stamps; I charge them $0.10 per page for printing. I figured out the cost per page (cost of paper and ink) and then rounded up to $0.10. I have a small printer and the ink carts don't last very long. It works for me, though, as I don't do a lot of it.