What is the cheapest you can buy prepaid phones?

I have a cousin who doesnt have a cell phone and she is constantly using my phone. I got in trouble with my dad today for using too many minutes... I know I don't talk on the phone that often so I know its her whose been using up most of my minutes. When I'm hesitant to let her use my phone she gets all snappy and angery with me. So I thought it would be a good idea to get her a prepaid phone for christmas. The thing is, I don't have an enormous amount of money, especially since I still have to get the rest of my fmaily presents. I checked walmart online and I saw one for $19.99, but I don't really understand how the minutes thing works. Would she have to buy more every month? How much do the minutes cost, and is it extra from the phone?