Many Americans think 5 cents a text message would be a bargain. Most pay 10, 15 or even 20 cents per txt without plan or on overages. It turns out the cost for these messages are next to zero.
The channel the messages are sent over is used to initiate a call. 160 characters does not exceed the amount of data used to start the call. The channel is left open regardless.
Telecoms: giving you less and charging you more.
Source: Wired blog