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.