SMS does indeed require its own set of hardware and communications protocols to support.

The hardware is the SMSC, and the protocols are a bit of a soup; primarily it's signaling over voice channels, but there is also SMPP. Your SMPP binds, of which there can be many, usually go over VPN tunnels. VPN tunnels impose load on firewalls, which can be fairly expensive.

Certainly, the cost of the hardware and increased load is nowhere near the charge per customer. But pricing is rarely based on how much it costs to produce a thing, but rather how much the people who want it are willing to pay. If they aren't willing to pay as much anymore then the pricing should go down.