Typically on T-Mobile plans, each text message over your plans allotment is 20 cents. So theoretically, you should only have gone over $0.40. Not bad. But this could vary by plan, and you could be charged more. I'd call T-Mobile to check to see your exact cost per message on your plan (you can get a hold of them by dialing 611 on your phone).
Sometimes carriers can temporarily upgrade your texting plan for the month to cover overages, too. You'd just pay the upgraded price. You can ask them about this when you call.
Sometimes carriers can temporarily upgrade your texting plan for the month to cover overages, too. You'd just pay the upgraded price. You can ask them about this when you call.