The difference between the least and most expensive smartphone is negligible (~$200). The real cost is the data plan, which will cost you $30 – $35 per month. So, a “free” phone will cost you $720 for the 2-year contract, assuming a $30/mo data plan. And this is also assuming you already have a voice plan with a non-smartphone.
If you can only afford $85 to buy the phone – and can’t afford a $200 phone – I suspect you probably can’t afford the monthly charges.