If you're able to buy the gold sink items, you're wealthy. As far as I know, the spider mount
is the current gold sink item, so if you have 2M+ you're probably well off in rich. I'd say anything between 2M and 1M you're fairly wealthy in that you don't have to worry about gold being a limiting factor in anything you do minus late game PvE whether you buy a run or buy BoE for progression raiding. Anything between 1M and like 200k is probably fine for most people. It lets them buy the items they need to play the game. Anything under 50k is probably at the point where you're actively looking to make money somehow because gold is a limiting factor of playing the game.
However, if you're in this subreddit and a goblin actively looking to make gold, anything under 1M is probably pretty low. I'd say something like 10M is probably at the point I'd call someone rich.