I switched to all-prepaid recently.
My data phone has been a T-Mobile $30 for 3GB phone for a while. But no voice and voice over IP sucks (e.g., Hangouts)
So I need another phone and it used to be an AT&T for $100 per month (they claimed it was $70 per month) and it came with 1GB which was horrible (though tons of voice).
I switched that to a $3 for 30 minutes and no data line from T-Mobile, and added credits as needed.
The last three months:
In short: it seems to cost me about the same for the voice. Part of the problem is a bunch of long conference calls. But $100 per month for pure prepaid vs $100/month for AT&T under contract etc etc.
I am thinking I should probably get one of those lame $40 unlimited data/voice carriers now. Will report back!