Update on 2014-01-29
It looks like I was off by a penny for the VOIP.ms fee. It’s really 1¢/minute, not 2¢/minute. This means that the overall calculation ends up being about 3.23¢/minute, not 4.23¢/minute. This is decently below the 5¢/minute for PlatinumTel. No idea if it performs well (in terms of latency and drop-outs).
Below is the calculation I did for myself around September of last year:
I switched to PlatinumTel on a pay-as-you-go plan. They are a T-Mobile MVNO. I like ‘em because they are pretty cheap (5 cents per minute, 2 cents per text), and also they have a pay-as-you-go data option (10 cents a megabyte). The pay-go data isn’t cheap, but we’re usually on WiFi, and it’s just nice to be able to get data if you’re in a pinch. (As far as I know, other pay-go operators don’t allow pay-go data option.)
I’ve been very happy with them. Their coverage is the same as T-Mobile, but (I’m guessing) they don’t support roaming.
From my calcs, the 10c/MB data rate even with an efficient vocoder (G.729) breaks even with the 5c/minute. And the latency isn’t that great. It may end up being cheaper, since I hear (no pun intended) that voice calls have a lot of silence, so you end up being ahead with the VOIP option.