Virgin is the cheapest no contract for the iPhone. I used Virgin Mobile for five years and had absolutely no complaints. I've since switched to Ting Wireless which is cheaper than Virgin but Ting does not, at the moment, support iPhones. Both Virgin and Ting use Sprint networks.
T-Mobile, AIO Wireless and Go phone can all utilize an iPhone purchased directly from Apple but will run $55-$70 per month or maybe more. T-Mobile also has a monthly payment plan for an iPhone. AIO and Go use AT&T.
I suggest Googling Straight Talk Customer Service. After I did that, I decided not to go with any of the variations including Net 10, TracFone, etc.