Ok, none of you are answering with an actual 'pay to go' answer.
The cheapest 'pay to go' option right now (and has been for a while) is Platinumtel.
$0.02 text / $0.05/minute voice / $0.01/MB data
Platinumtel is a Sprint MVNO, so it's using Sprint tower coverage (similar with Virgin Mobile, Boost, etc). This would be a definite no for some people, and ok with others. The $10 pin is good for 90 days.
The only bad thing (and for some people it's a deal breaker) about this is that they have a rather crappy phone selection. But if all you need is voice call and some texting and you don't use your phone for more than 100 min/month then there's nothing better than this.
Edit:
I just wanted to add that there is a Platinumtel dealer out there that will help you BYOD any Sprint phone to Platinumtel (exception is iPhone and Blackberry), but there's a $40 fee involved with it.