I just checked and I've spent almost exactly $60 on my pay-as-you go phone since mid-October. That comes to about $15 a month. I make no effort whatsoever to reduce my phone usage, other than the fact that I don't enjoy talking on the phone at all. I use AT&T, so I pay $1 a day when I use the phone and then it's 10 cents a minute for calls outside of the network. As it turns out, most of the people I talk to have iPhones, so I'm not getting charged the 10 cents per minute. I also buy the minimum amount of text messages every month (200 for $5). I've never used all of them and they roll over each month, so my current balance of text messages is about 475. This may not be for everyone, but it's working for me. My phone is also the 'dumbest' phone I could find. Cuique suum.
If you can't rack up $60 worth of calls in 1 month on PAYG, that's amazing.
Assume: 10 cents/minute
A budget of $60 gives you 600 minutes/month. That's 20 minutes/day in a 30-day month. If your cell phone is your only phone, like it is for me and for many others, it's very easy to breach 20 minutes/day. Especially if you have a significant other.