hi. Have you looked at carphone warehouse or somewhere with her? They have good websites and I know you can get some decent cheap girly pink ones! Im sure my SIL has one that she said was £70 when I told her I liked it!
Personally I think having a mobile can teach independance and budgeting as it can be tempting to call or text their friends ALL the time! However, many schools (including the one I work at) have now banned mobiles from school and will confiscate as there have been increases in phone related crimes on buses etc. I would just have a word with your daughter about keeping it hidden as much as possible. It is a sad fact that pay as you go can be so easy to steal! Good luck! Xx