As a Professional Photographer and Retoucher, it should NOT cost less than $5 to retouch a photo unless you are doing very high volume minimal retouching. It takes about 3 minutes to retouch most photographs on the basic side, longer with more advance processes.
What Sells Best On Shutterstock?
Some types of photos that sell best are Portrait Photography, Food Photography, Landscapes (usually with people) and fine art photography (artsy).