Can an iPhone phone bill be just $50?

So my friend wants to get an iPhone 4 and she thinks the phone bill will be only $50 with the data plan and unlimited talk and text.she's with t-Mobile and honestly I think she's gonna have to pay somewhat in the $100s for her bill cause that's what my dad pays and he barely uses his iPhone 4 but he has AT&T… So can I prove her wrong and say its not gonna be just $50!

Close to $100 with T-mobile.

Should be more than $50 with a data plan!