I have T-Mobile. Phone is about $50. Start with a $100 card (so you get locked in to a 360 day expration date) and then you can refill in the increment you need ($25, $50, ect)
At the $100 level,its about .10 cents a minute ($25 refills work out ot about .17 cents)
Its perfect if you only need the phone for emergency use.