I have an iPhone 4S with AT&T. I paid $100 for the phone with a 2 year contract, and have 450 minutes, 300 MB data, and pay-per-text. It's normally $60, but I get a ~$10 discount through my employer.
DH just switched to a T-Mobile plan that's only $30 per month- no contract, unlimited data & text, 100 minutes. Since he doesn't use the voice feature often it works well for him. I'll probably switch to it, too, once my contract expires. He bought a Nexus 4 directly from Google for $300 (which is about $130 cheaper than buying it from T-Mobile)