I recall reading somewhere that SMS messages get sent as part of the network synchronisation messages that keep your phone connected to the nearest base station. As such, there is no additional cost to the network for sending and receiving them, only in carrying them across the network, and passing them on to other networks. The cost to the telco of carrying the messages, as opposed to not carrying them, using the infrastructure which is already there, is as close to zero as to not matter.
Biting the hand that feeds IT
