i think you should let her go if she pays for it. she is 21 years old so its a chance for her to show some responsibility. im not too much older than she is and i know my parents would let me go only if I paid for it. She is close to being on her own soon, so the last thing you want to do is not allow her to do something she really wants to do. It could hurt your relationship with her if you dont let her go.