There are tons of plans if you go prepaid as others have said. You have to buy your own phone, but that is a $50-200 investment. If you get an android it is cheap, but an iPhone will cost you hundreds more.
I have virgin mobile, at 2.5GB they cap your data at 256kbps. Netflix eats through data fast. Even at the lowest quality setting (which is perfectly fine for viewing on a phone) it eats up 0.3GB/hr. So in 8 hours you have hit your cap.
However I was able to continue streaming netflix at 256kbps. So there is that.
Do you live in a large city? Would you use your phone mostly in areas with wifi (home, restaurants, etc)? Wifi will not eat up your data.
The reason I ask about a large city is because Virgin Mobile has better 3G coverage in smaller towns than T Mobile, but T Mobile has better data speeds within large cities. It is a trade off. Since I use my phone outside of the city a lot I picked Virgin Mobile.
If you are set on an iPhone, you may be able to get a refurbished iPhone 4 for about $200-300.
http://www.virginmobileusa.com/shop/cell-phones/
I used to have a dumb phone (tracfone) and an ipod. Then I upgraded to a smartphone. It was one of the best investments I’ve ever made for the money spent (about $20 more a month than my dumb phone). Totally worth it.