Why do you want to buy off contract?

T-Mobile's cheapest plan is $50 a month (before taxes) for unlimited talk and text, 1 GB of LTE data.

Then they must have some kind of installment plan to break up the payments over a 2 year period so perhaps another $30-40 a month until you pay it off.

Main reason to get T-Mobile is for the free international data roaming. Otherwise, AT&T and Verizon likely have better networks in most places in the US.