It's anywhere from 30-65 echoes per arena win. I don't have weapons for frost, but somehow I was able to beat a few teams with unholy. It wasn't a particularly fun use of time. I really wish they'd bring back 5v5 arena. 3s could have had some potential as well, but UH just has no burst, insta-gib potential.

Doing arenas are definitely faster than grinding mythic+ dungeons, if the sole purpose for echoes, but as you point out, there are a bunch of other, seemingly better ways to grind echoes up from the daily/weekly activities. Unfortunately I'm a little stalled out on my DK team until I get some Twilight Devs rolling.