Prepaid services vary widely depending on the country you're in. In some places, the credit is valid for a quite short amount of time. In France, the validity of my credit depends on the amount I charge. So I had to make some crazy calculations to choose between putting around $10 that will be valid 15 days, or $25 valid one month, etc.
On the other hand, I'm in Sweden now. My phone bill for the whole year has been below $100 (I don't phone much, but I use internet a lot). So, "pre-paid" can mean a lot of different things.
Free in France is now selling a monthly subscription you can quit anytime (no contract, no subvention for the phone, just like the typical prepaid stuff) that gives you both unlimited voice calls and unlimited data and sms for only 20 euros. 16 euros instead of 20 if you're also subscribed to their ISP.
And for those who don't really have a need for cell phones beyond the basics, they have an offer at 2 euros a month for 1 hour of voice time and 60 SMS. Even better, it's free as in beer (0 euros) if you have chosen Free as your ISP.
http://www.free.fr/adsl/index.html
You'd really have to be bad at math to chose an expensive contract nowadays over paying in full for your own phone and getting that kind of plan.
On the other hand, I'm in Sweden now. My phone bill for the whole year has been below $100 (I don't phone much, but I use internet a lot). So, "pre-paid" can mean a lot of different things.