So I tethered my phone to my laptop today to play some XIV while at work (I work nights and have a lot of time). I was interested to see that after 45 minutes of dinking around, killing stuff, and being rather random I only transmitted 6MB out to the servers and received 5MB. That includes zoning from Ul'dah to West Thanny, doing some killing, and some mining.
That's rather small. I didn't see any FATEs in my area so I didn't see what that does to my data usage but I'll give it a try and see what I find. Overall this game is extremely tether friendly from what I can tell. I can only imagine that instances uses less data.