I went to London for a conference in October 2005. I topped my PAYG mobile with �20 which gave me about �25 of credit. I had to top it up last week as I was down to less that �1. So I used about �25 of calls in 9 months. That works at less than �3 per month.
When I bought the phone about 4 years ago, I was laughed at by people who couldn't believe I chose PAYG. They told me that for just �10 per month I could get hundreds of minutes and loads of texts (which I never use). So I guess the last laugh is on them.