I'm not saying this with any certainly, just suggesting that it's a possibility: in the early days of cell phone service, people had to pay a fixed amount for each SMS sent (and, I believe, received as well)--perhaps $.1, or so, per message. Then, a few years later, you could pay for more expensive plans that included a certain number (some variants included baseline plans with a la carte options). Eventually this was abolished, in favor of plans that included unlimited SMS (the proliferation of HS Internet access probably had a lot to do with these changes as well).
However, it is also true that when it comes to commercial accounts, pricing for certain options like SMS may still follow the original scheme--i.e. the telecom may charge a fixed amount per individual message. I'm not saying this is definitely the case, nor that it is reasonable, if it is the case (the contention early on was that SMS were part of some other communication that occurred automatically, so people should never have had to pay anything for this "service," which was periodically ongoing by default). I'm just pointing out that this is how it used to be for private accounts, and i wouldn't be surprised if it was still the case for commercial accounts.