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.
Best Answer
You're asking one of the hardest questions in the industry. Tabletop game publishers are very reluctant to release firm figures, being very skittish about appearing as 'weak'; this problem has plagued the industry since I got in in the early 90s, and doesn't appear to be going away anytime soon. Also, the numbers you report are self-reported, which is not ideal. However, we can infer that it's a tiny subniche of a niche market from several different sources:
First, ICv2, the trade magazine of the industry, has reported the total volume of trade done in the RPG industry is $25 million in 2015. (This is actually an increase of $10 million over 2014.) No matter how good an OSR game does, this is the upper cap.
Second, no 'indie' game is in the top five.
So, if they're not breaking ground in the major games market, let's check RPGNow (another brand of DriveThruRPG); OSR games like Labyrinth Lord have made major headway there. Sorting the OSR category by popularity, we see that the best seller for a game tagged as OSR is OSRIC Unearthed, which RPGNow marks as an Electrum Seller. As an Electrum seller, it has sold at least 250 copies, but not 500 copies (to make Gold).
Finally, let's check their top 100, which scales games over time. The highest-ranking game is Guardians, #29 in the Hot 100. What does that mean? Well, this blogpost by 6d6 Games reveals that their game, which reached the position of #13 in RPGNow's Top 100, made it up to 38 sales. The numbers are not directly comparable (since it's sales-over-time, and the figures are from different years), but it illustrates the size of the OSR market.
That said, if you want to make your OSR games, there is an alternative - crowdfunding. The most successful OSR game on Kickstarter right now is The Black Hack, which reached 604 backers.
So no matter which way you cut it, us old-school murderhobos are a rareified breed.