IIR the 140bytes is bandwidth already used by the carriers, but the messages do require store-and-forward servers. By now the servers have to have been paid for, so the carriers are making almost pure profit, except for maybe a little upkeep on the hardware.
Sales strategies aside, I see no reason why texting is still so expensive or even charged for at all. The technology is mature and in maintenance mode. Carriers seem to be milking as much profit as possible before they have to compete with the data versions.
Of course, if they have their way they won't have to really compete. By capping data plans to point it's not worth it, they can still keep some control. What happened to the days when "unlimited" meant unlimited and not "unlimited but only as far as we so"?