The only scarcity is in your cell, right now. In 99% of cells, txt messages are free as the signal time slices are going unused (no other texters, low signalling use). At, say, a rock concert, the wireless tubes might be crammed but that's why SMS doesn't have a latency (or delivery?!) guarantee. Also, a mobile base station could be set up at the concert to get more airwaves or there may not be enough voice channels.

So... txt messages are essentially free.