I don't agree that pricing should only offset your costs. Photography was a hobby before you started making money with it, so the cost of your equipment isn't exactly an expense of your photography business.

Instead, I think the end product you produce is the only factor in pricing. There is no reason for a client to have to pay for your expensive DSLR if you output photographs that could easily be made with a point and shoot.

So a good place to start is to look at other photographers and studios that output similar products (senior portraits) and have similar quality to your pictures. Then check out their prices and maybe take an average. This is a good place to start for sitting fees.

When selling prints remember that you aren't just selling the paper and ink -- your artistic talent, along with time dealing with printers, also went into producing that print. Don't sell prints at cost. This is also another place where it is a good idea to look at other photographers and studios for guidance.

Try mirroring prices of a nearby studio that does the same thing you do. After a number of sittings, evaluate how well the prices you choose reflect your work. Don't sell a valuable photograph for less than it is worth (called undercutting the industry and this is what makes 'pros' get so insecure and start to dislike you), but also recognize that you don't take the same photographs that a top-tier studio can.