My friend bought some pictures of her from a photograher and 8x10 for $15. I don't know though, I"m sure it depends on what it's a picture of, who took it, and what size the print is.
Lots of things to consider... What printing service/company are you planning on using? How much are they charging for the specific size--what's your margin of profit? Is this is a matted print? Framed (if so, factor in cost and profit of overall picture)? It's not an easy one-size-fits-all answer..
A photo of what? Printed at what size? On what type printer and what type of paper?
Has the photo been edited? Are you a photographer that has some renoun? Or are you an unknown photographer?
Lastly, pricing photography has little to do with 'being fair'.