What are the best Pay as you go mobile phone plans?

I currently put it Miami, Florida. I've been using virgin networks since last 8 months. One of my friends informed me about pay as you go mobile phone plans. I really liked the scheme and now I am planning to switch on this plan. But before I do anything in concrete I want to know about the terms and conditions to have this scheme updated? I spend $79 for the latest plans I use, do I have to pay extra bucks for this plan? Is going to be an expensive plan for than the one I am using right now? Please help me by providing details regarding the best Pay as you go mobile phone plans. Thanks.