How much would I pay for an iPhone at sprint?

My mom has a plan with 2 iPhones a galaxy s3 and a katana phone. I have an iPhone 4 and I want to know how much it would be for me to branch off from my moms plan and just pay for my own phone. I want to keep the same phone and number.

My mom is switching companies and I want to stay with sprint.
I'm going to have to pay for my own phone, I don't want it to be more than like 50-60 dollars a month. Also I will get a 5% discount because I work at ace hardware.

I think I saw the iphone 5 for $100

With unlimited data, calls, and texts it's at $79.99/mo.