Start off from the cost of making the print. Total cost to the customer shouldn't be more than twice that. That's a general rule of thumb: if you were using a service such as Cafepress, you're paying them both for making the print and doing the order processing. And there's also a cost in having a stock of prints ready to mail. But the really obvious rule is to cover your costs: not just making the print, but the costs of postage and packing.