Lets make a deal. I’ll massage your back for the rest of expansion while you farm gold ON YOUR ACCOUNT OF COURSE and when you hit 5mil you’ll mail it to me.
I have no idea if its against tos.
As a poster said, boxes are called ‘Unclaimed Black Market Container’.
This boxes contain various items: rare mounts, not longer able transmog, rare recipes, illusions, etc. Every day you’ll see a box able on your Black Market Action House.
Items you get are not based on for character class/armor type (there’s a possibility you get a extremely rare transmog but not the appearance because it’s for an armor type you cannot equip) nor on your collection.
Prices are between ~200k and ~500k depending on your server population and connected servers.
The not so rare mounts such as the Core Hound from the 10th anniversary goes for around 500k, affordable for some casual players.
But very rare mounts like the ZG tiger? Pft good luck on that. Everytime it shows up on the BMAH it instantly gets sniped for gold cap so nobody else can bid on it
Even if you do have 10 million gold you will still need to click on that bid button faster than light before anyone else.
If its just for Ashes i would just suggest for you to farm it. On multiple characters if possible if not just create and level them. To answer your question like many before me have said you can get it for a few hundred thousand or for several millions, its rare to see it on the BmAh. Also im not sure you could buy BfA atm but if its possible just buy Shadowlands you will get BfA for free with it. And if you are only considereing to buy an expansion because of the BmAH i would advise you to just pass on it.