As long as grades, family time, and normal interpersonal relationships are not suffering, I think it's ridiculous to overreact to how many texts a teen has racked up in a month. What difference does it make?? Unless you are SURE that she is making every one of them and spending the amount of time you have estimated, I think you should back off completely. Unlimited texting is not that expensive and you are not judging her actual activity correctly. What if half of those texts are received? It doesn't take more than a few seconds to read a text and hit "erase", and the normal teen doesn't need to spend the amount of time you think texting - they are very fast.
I don't think the replies that say "Good for you and you need to crack down now!!!" really know what they are talking about.
Trust me, there are MUCH bigger battles to fight and why a parent of an otherwise healthy and happy productive teen would want to get in this battle is beyond me.