Jenna Wortham writes for the New-York Times:

Professor Keshav estimates it costs the carriers about a third of a penny to send text messages. Considering that the major carriers charge 10 to 20 cents to send and receive them, “it’s something like a 4,090 percent markup,” he said. At 20 cents and 160 characters per message, wireless customers are paying roughly $1,500 to send a megabyte of text traffic over the cell network. By comparison, the cost to send that same amount of data using a $25-a-month, two-gigabyte data plan works out to 1.25 cents.

With services such as Blackberry Messenger, iMessage, etc., the number of texts sent will decrease so as real phone calls in the future. Full data plans for mobile phones make definitely sense to me.