It cost ~$1 (initially a one time fee, later joiners paid annually). Depending on your SMS plan, that might be paid for with the first 10 messages you sent. There was zero additional cost to use to send a lot of messages, including pictures and group messages. This was quite different to most mobile contracts at the time

It wasn't even enforced though. Charged only after a year, but you could reinstall and get another year free.

I've used it at that time, and somehow they never managed to actually charge me (and I didn't go out of my way to avoid paying).