Depends on where you purchase the frozen rodents from. I purchase mine from a local Ball Python and Rodent breeder who also owns a reptile store. The size of the prey depends on the size of the snake. My male and female normals are the same size right now, I feed them both small rats which are about $2.20 each. They eat about once a week, so for one of them I pay $8-10 a month.. really not very expensive, in my honest opinion.
Btw, I strongly recommend sticking with the tub setup to house him in. Display tanks are wonderful, but from experience they are a pain in the butt to maintain proper temperatures and humidity. Also a serious pain in the butt to clean! I switched to Sterilite tubs and never looked back!