I came across this post on the blog Monsters & Manuals about the campaign setting Yoon-Suin and its units sold since being published. It's interesting to read for someone who's entertaining the idea of publishing something themselves, but it leads to a few questions. Yoon-Suin is near the bottom of Page 7 (out of 10 pages of 50 items each) on Drive-thru RPG's website when you search for things tagged OSR and sort by popularity, but I'm not sure how linked popularity is to actual sales numbers. It leads to a few questions about the average sales of adventure modules and campaign settings.
What is the average number of units sold for modules and settings in the OSR (separately listed if possible)? If these numbers aren't known directly, can they be ballparked?
What about the average units sold for relatively well known publishers in the OSR, such as Sine-Nomine, Goodman Games, or LotFP? If these averages aren't known, can they too, be estimated?
If any of these numbers aren't known and can't even be estimated, why can't they be estimated?
I want the units sold, not necessarily the gross or profit made. I'm well aware this is a niche of a niche of a niche and am primarily concerned about how many people these products reach as opposed to how much money will be made. I'm also interested in what factors cause units to sell more (better known publisher/author, art, system compatibility, price point) although these are nowhere near necessary for a good, acceptable answer.
The more specifics offered, the better; ideally an accepted answer would contain answers to all of the above and the rationale behind said answers.