The New York Times talked about the pricing of TXT services and the cost of delivering the service. To elaborate on what the article says, the marginal cost of sending a TXT message is 0 (it is a heavy fixed cost operation and the opportunity cost of capacity occupied by a single TXT message is 0).
That said, why does cost matter in pricing? Cost is irrelevant to pricing. It is understandable that carriers price it based on what the market is willing to pay. If the price is high the usage of TXTing will shrink and/or customers will switch.
It is not a good idea to have congressional oversight over pricing a service or dictating that price reflects the cost. Of course this will lead the carriers to artificially allocate their fixed cost overhead to every single TXT message (like government contractors do to ) just so the price would look closer to cost.