Let's talk numbers. Hypothetically, we have a 300 page game book.
A "standard" manuscript page is considered 250 words. That means, from my own experience with Wizards, one page in a game book averages to three "standard pages."
So we're starting with a figure of 900.
This a very technical book, so we'll put it at the bottom of the "standard" range for proofreading. (We're calling it proofing because we're talking about a late, "find lingering errors" pass; it'd probably be heavy copyediting or developmental editing earlier in the process.
Which puts the rate at 9 manuscript pages per hour.
We're not talking about a ten-hour process, but a hundred-hour process. (And before you tell me that's ridiculous, try reading a book like the PHB looking at every word for errors.)
Proofreading at that size is likely to cost you about $35 per hour.
$3,500 may not sound like a lot, but it actually is a measurable chunk of the cost of production. More to the point, it's $3,500 being spent to reduce the typo rate by, in the examples being discussed here, a fraction of a percent.
It's not worth it from a monetary standpoint. A .3% reduction in typos is not going to bring in extra sales--certainly not $3,500-worth.
(All rates/figures estimated via the Editorial Freelancers Association.)