I'm using the Progallery (http://www.photoboxgallery.com/take-a-tour) part of Photobox which isn't bad at all actually. The costing is their price to print plus 10% of your actual price.
For example, they charge £1.27 for a 10" x 8" print, you sell it for £5.00 (you can either use a % mark-up...