You are charged 5¢ each for all incoming text messages whether you open them or not.
If you're getting a large volume of unwanted text messages , i.e. "spam" then call SO customer service. They may be able to do something, e.g. offer you a new number or maybe block all text messages for your current number.
This site is run by a 7-Eleven Speak Out Wireless customer. I am not a 7-Eleven employee, nor do I receive any compensation from them for providing this site. The goal is simply to share consumer information with others.