I just come up with a standard number for a given size. There's also a world of difference between the why of pricing for a print sold to a portrait client and an editioned print. With "commissioned" prints, there will always be some that you'll lose money (or at least break even on) because they're hard to print. But that's why you do your proofing at small sizes and take good notes . I'd say figure out what the "average" print you make costs you in terms of time and materials: put a value on your time (an hourly rate), multiply your hourly rate by the time it takes to make a print (and by that I mean account for every second from when you pull the negative out of the sleeve to when you put the print in the final wash), add up your materials cost, double that, and add it to the hourly price. Don't forget to include your chemistry, electricity and water costs in he material cost.