How much will my next sprint bill be?

I have the $69.99 monthly plan, this will be my second sprint bill for my iPhone 4S

Its $79.99 (the 4s is considered a smart device and they charge $10 more)
and then there are local service fees and taxes which vary
you could look at your last month's bill and likely it will be similar, or the same, or you can log into your sprint account online and access your bill.