@variablepulserate MMS for me is included with unlimited all the things on my MetroPCS plan AFAICT. (I do have a 2 gig data bucket, but I haven’t had to worry about MMS with it yet. I’ve exceeded it once in a few years. I’m on wifi 99% of the time.)
I can’t remember how AT&T handled it.
I mostly gave up on MMS when network peering brokenness made it so none of my messages made it to many of my family members. Turned out Republic Wireless just doesn’t interop for MMS with some providers. I was sending messages into the void. I switched to Signal instead.