From https://support.apple.com/en-us/HT207006 (at the bottom of the help article) - "If Wi-Fi is unavailable, iMessages will be sent over cellular data. Cellular data rates might apply." So I'm not sure if it is free over mobile data.
>On the other hand, I believe AT&T zero rates their RCS messages if you have an unlimited texting plan
But what if you don't have an unlimited texting plan? Let's say your current plan includes data + modest amounts of talk+text, but you do most of your communication over chat apps, so you never really touch your text allowance, even though you text a lot. now with RCS, you now have to pay extra $ for those texts you're sending out, or pay $ for the unlimited plan. plus with chat apps, you could send free over wifi. so from a customers point of view, RCS is worse in every way.
On the other hand, I believe AT&T zero rates their RCS messages if you have an unlimited texting plan. (e.g. https://www.att.com/shop/wireless/features/advanced-messagin... > FAQ > how much does advanced messaging cost). Could not find details on TMobile except this article: https://newsroom.t-mobile.com/news-and-blogs/rcs-messaging-l...