Comment Don't forget to factor in the SMSC (Score 3, Informative) 570
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.