What you would be charged would depend on your plan. Unlimited texts are included in all current plans. Some old grandfathered plans are charged per text if you do not have a text add on for your old plan.
Getting messages that you do not want can be a hassle, and we want to make sure you get all the information on how to stop them. The message that you get stating that you may be charged is set in place for users that have a set number of text messages. If your plan has unlimited messaging, no charges will apply. Does this information help?