Agreed. I knew what the text messages cost when we got our son a phone.
My mistake was thinking "he won't use more than that..."
When my son got his phone, he stood with my husband in the store and insisted that he didn't have anyone to text. My husband explained it's only $10 per month, no big deal. Nope, don't need it, he said. My husband said ok, fine, IF you end up using it, just let me know and I can add the unlimited. I think there was some small number of allowed texts with whatever plan he had.
A month or so later, the kid comes sidling up to me. "I've been texting on my phone and I probably went over the limit. What do I do?" Go confess to your father before he gets that bill!
Luckily, he hadn't gone over the limit by much, so it ended up not being a big deal.