In this article from the NY Times, we get a comprehensive explanation of how the economy of SMS/text messaging is clearly a price gouge, and that the idea of message numbers going up does NOT indicate proportional cost increases for the providers.
A few key quotations:
"All four of the major carriers decided during the last three years to increase the pay-per-use price for messages to 20 cents from 10 cents. The decision could not have come from a dearth of business: the 2.5 trillion sent messages this year, the estimate of the Gartner Group, is up 32 percent from 2007. Gartner expects 3.3 trillion messages to be sent in 2009."
"The lucrative nature of that revenue increase cannot be appreciated without doing something that T-Mobile chose not to do, which is to talk about whether its costs rose as the industry’s messaging volume grew tenfold. Mr. Kohl’s letter of inquiry noted that “text messaging files are very small, as the size of text messages are generally limited to 160 characters per message, and therefore cost carriers very little to transmit.” A better description might be “cost carriers very, very, very little to transmit.”"
"Srinivasan Keshav, a professor of computer science at the University of Waterloo, in Ontario, said: “Messages are small. Even though a trillion seems like a lot to carry, it isn’t.” Professor Keshav said that once a carrier invests in the centralized storage equipment — storing a terabyte now costs only $100 and is dropping — and the staff to maintain it, its costs are basically covered. “Operating costs are relatively insensitive to volume,” he said. “It doesn’t cost the carrier much more to transmit a hundred million messages than a million.”"
"Once one understands that a text message travels wirelessly as a stowaway within a control channel, one sees the carriers’ pricing plans in an entirely new light. The most profitable plan for the carriers will be the one that collects the most revenue from the customer: unlimited messaging, for which AT&T and Sprint charge $20 a month and T-Mobile, $15."