My S390G did something similar, charging me .6 for a text reply. My guess (and I could be wrong) is that if a text is longer it incurs the longer charge, something that might be more likely given that a reply includes the original message in it.
Given how Tracfone has quietly increased data rates on their 3G phones, this is probably not a surprise. (Maybe that's why these phones are TMFL?) Irritating, though.