What is the best mobile tariff for my teenage daughter ....?

My daughter, like many teenagers is on the phone night and day. We like her to have a phone as we feel she is safer out and about if we can contact her. We started off with a monthly contract but she consistently and quite significantly overspent so we switched her to a pay as you go. Now she has generally run out of credit half way through the month. However she complains that pay-as-you go works out far far more expensive. Has anyone heard of a monthly contract that is somehow 'capped' and you can't go over a certain limit?