The cost involved in transporting text messages is not just the capacity in the network. These messages all end up on a SMSC, a carrier-grade system able to handle multiple-hundreds or -thousands of simultanious SMS messages and route them to other subscribers and operators. These systems are provided by a handful of suppliers that know what to charge for a decent cluster of these baby's... and somehow they need support as well!
NB. Not in any way affiliated with telco's.