Let's how PoE:TWM fares in comparison. Most popular DLC achievement is :
3.2%
Soulbinder
Unlock all the powers of a soulbound weapon.
Roughly 21,316 players. You do get a couple of them very early in the DLC, so I'd say it counts.
There's always the group of compulsive buyers that want to have all DLC before even starting a game, and those who bought this after finishing the game but have yet to touch it. Only multiple devs could tell us the average % of players who buy and actually play DLC content like this.
Still, it's worth reminding that only 9.2% finished the game, or ~61,285 players. Compulsive buyers aside, which should be a relatively small number, it takes satisfied players to purchase more content 6-12 months after launch. I'd say it'd be unreasonable to expect a number higher than the amount of people who finished the game.
That said, an educated guess would be somewhere between 25,000-60,000. Best case scenario,
everyone who finished it bought the DLC (hundreds of codexers didn't, but it's just an exercise) + compulsive buyers, so ~70K. About 3.28 times the number of players who got the earliest achievement in TWM.
Using that same metric, the best case scenario for SoD would be 13,257 copies sold.
However, SoD is a different beast, as a lot of people finished BG in between months and 17 years ago and were only interested in the expansion. Even if you double or triple that number, it'd reach the overly optimistic 40k I mentioned earlier.
Disregarding regional prices, the revenue from 13,257 copies would be $185,598, while 40k copies would give them $560,000.