No-contract is 50 dollars for unlimited everything ($70 if you want 4G): http://prepaid-phones.t-mobile.com/monthly-4g-plans
With a contract it is 90 dollars for unlimited everything (which I believe is roughly standard across the industry): http://www.t-mobile.com/shop/plans/individual-plans.aspx
http://opensignal.com/coverage-maps/US/San%20Francisco/
T-Mobile in San Francisco makes AT&T's network look good and it's a shadow of Verizon everywhere.
Is it really, though? As an example, T-Mobile prepaid offers $30/month for unlimited data (5gb @4g) & SMS, plus 100min voice. Poking at ATT's site the most comparable contract plan I see for iphone 5 is about $90/month. That's over $700 more out of pocket in a year if you go with the subsidized phone.
That's the same savings you get with an iphone on Virgin Mobile/Cricket though [1]. Very few people do this though because Sprint's network sucks and the upfront cost is a turn off. In addition many people do not pay the full plan rate. My monthly AT&T bill is around $59 including fees.
[1] http://news.cnet.com/8301-1035_3-57449345-94/why-a-prepaid-i...
You also can't ignore the fact that in the US other phones in it's class are subsidized by $400 or more, upending this comparison greatly. Yes you're locked into a contract but it's not like Nexus 4 users get some special monthly rate. For some use cases being off contract is a god send, for most though, a subsidized iphone 5 or galaxy s3 is a better option at $100 cheaper (and a higher resale in case you want to break that contract).