Brandnewday is right, kids do appreciate their things more when they buy it themselves. It also teaches them responsibility before they get out on their own.
My daughter is a senior in school (18 yrs old) and works part time. She has been paying her car payment and insurance payment for over a year. She won't let me help because she said it is hers. She also buys most of her own stuff. It has taught her the value of a dollar. I have tried to pay for things when I can but she won't let me.