It was expensive before because it's a supreme form of communication: don't need both parties to be online and vacant, conveys complex info in a succint form (think numbers, adresses) that's saved on both parties' phones like notes for later, have an option to pay for another party to answer you, could be printed and sent in secret when there's a meeting or a lesson (with physical buttons most could've printed a message under the table) and it kinda had it's own culture, with basic emoticons and shorthands for words - even if being far from how popular it became in Japan and in pager's times.
Right now SMS messages aren't used that much by individuals since it was killed by the internet+messengers+touchscreens trio so they charge extra to milk those who happen to need that. A lot of carriers at my place provide subscription plans with minutes and gigabytes, with SMS as additional paid package or with a ridiculous price for every message.
But they don't actually earn much here, even if they charge a whopping lot. SMS providers get fed by commercial contracts with services who notify you of delivery, send you verification codes and show you your CC balance. Signal, the messenger, started to look into phone number-less accounts because their non-profit is tired to shoulder the price they need to pay for just an SMS per login. Some services now call instead of texting you and make you write the last numbers of the phone to verify your identity, because it's cheaper for now.
I feel kinda nostalgic of times when it was popular and carriers introduced special plans for messaging enthusiasts. I hate people calling for every small thing and I'm tired of checking multiple internet channels of information. With SMS it all was simple and direct.