Not on AT&T, as far as I can tell. It's either $20/month for unlimited messaging, or $0.20 each. So, unless you're sending or receiving more than 100 messages a month, you're better off without a plan.
I've got a 200 message/month plan from AT&T for $5 a month. My only complaint is that I can't share messages across the two lines on the account so each gets billed $5. (Also, no data plan.)