I agree with the post (whoever it was) that said how come she suddenly doesn�t like it now that she has found out it was �12? If she liked it, and everyone was going woo, you lucky girl, that is a really nice ring, must have cost a fortune etc etc, he is unlikely to blurt out that it was a cheap purchase is he?
He was probably smiling safely in the knowledge that all the women were convinced that the ring was expensive. It is unfortunate that he kept the receipt in a place it could be found, but my wallet has receipts in it which are 6 months old. He has been caught out and if the girl is truly upset maybe she should let him know.
Getting revenge as mentioned above would just make her look very very shallow and materialistic. If my wife bought me something that I really liked, I wouldn�t care if it came from a 99p shop personally.