a lot of people who don't really need a phone use their pay as you go service. it's expensive if you use the phone, it's the cheapest you can get if you only need one for an emergency. like you probably know since you asked about sprint, it's just resold sprint network.
boost is currently offering $50 unlimited minutes. the phones are expensive though if you buy one new b/c there's no subsidy.
if you plan on using your phone like you currently do, then i wouldn't recommend pre-paid virgin.