Not sure how much this really goes for but after the long shadowmourne quest chain, I was very surprised to find a BOE tabard of lightbringer as a reward…only one on AH for 750k/1million so I can’t really tell how much it would sell for if I sold it(and ran it again on warrior for a personal one).
It’s certainly not as valuable as it once was, just due to the fact that most of the quest can be soloed or at most, done with 2 people and an hour to kill for the infusions. However you can easily still get a couple hundred thousand for it on populated realms.
You can put things on the AH for any price you want, but I promise you that the 750k one will never sell. Personally I got mine for 100k (on Sargeras) and I think that’s a pretty average price. Check the Undermine Journal for your realm, but I wouldn’t expect any more than 150k-ish.