And now you also know that any carrier charging you more than $0 for SMS is full of shit (it doesn't cost then anything)
Yes it does cost them.
You are perpetuating a fallacy, that fixed costs don't matter, only marginal ones per transaction; this is the same sort of flawwed argument some people use to rationalize music or software piracy to themselves.
The reality is that in business, all costs matter, and products in the marketplace get priced based on both fixed and marginal cost.
The marginal cost to the carrier per SMS message are very low or close to $0. There is no additional cell network capacity used.
On the other hand... what is consumed, is capacity of systems involved in storing and forwarding the SMS content.
That is to say, when you send someone a text message, there is an entry in a database created somewhere -- that record needs to get from your tower to the recipient's handset; there will be some cost in terms of magnetic storage.
If they are purchasing enterprise class storage arrays for branch locations where they route these messages, the average cost is about $25 per gigabyte.
at an average length of 80 characters per text message, and each message passing through each system, each text message costs about $0.000004, in storage that will be temporarily tied up; now, each processor node that receives and forwards these text messages, also has a CPU capacity, and each text message has a fraction of CPU and RAM that will be temporarily tied up as well.
When all is said and done, you can make the argument that a SMS usage, probably takes up $0.0001 in marginal cost.
Furthermore, there is some equipment the carrier has to purchase and continuously maintain for SMS functionality to continue to work. They also have to provide support for their customers, so there is an average operational cost text per message per month (in support terms) for providing a SMS service.
The average fixed cost portion, eventually decreases with sufficient number of text messages -- at least until equipment capacity is reached, and better storage, forwarding, and accounting systems are needed to provide more capacity -- stair step pattern, if you need to buy a $500,000 storage array, the cost per text message ever sent will initially will be very high, and gradually average down over time.
They charge users of the service more than what it costs them, per message
That's called margin, and is a fundamental requirement for a service to be worth providing -- if there's no profit in it, then the carrier should not provide the service. And you could make the argument that they are taking advantages due to the lack of competition in the current market place, resulting from monopolistic practices, and the government's anti-consumer practice of auctioning "exclusive spectrum rights", to supplement the treasury's tax revenues.
However The cost per SMS message is not $0.
The marginal portion is zero. The fixed portion is not.