Cost is something only you should price - don't ask other people about your own prices when running your own business unless you know them to be very professional or you're paying them for consultation under contract.
However, I'll give you some pointers to expand on what was said above.
When pricing your accounts you obviously need to run at a profit. There is really no such thing as nonprofit hosting though many companies try and live by it these days. You need to consider your own resources. Typically I take my alloted transfer (transfer and bandwidth are different) and figure up the cost.
IE:
I have 50 GB of transfer and I'm paying $15/month. Doing the math there would give you a price of $0.30/GB. Obviously you need an amount higher than this to profit reasonably for your business. You can either oversell for lower prices (which can get you into trouble if not properly monitored) or you can sell what you have. Either way you need to figure out a plan where you make the most. I'd look at charging no less than $0.60-$0.75/GB here (for example and depending upon your business plan) or higher. It all pretty much hinges on your business plan with how much to charge. Do you want to charge more so you can provide better support or do you want to run at cheap prices which involves more risk?