The 15% offer came before the leaks, incidentally, that was their opening gambit to have 25% the standard withb15% for negotiated contracts.
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Even assuming more knowledge than I am guessing the beers guys had, let's take a look at Kobold Press Tome of Beasts line. They had fewer than 10,000 backers for the latest one on Kickstarter, and made some thousands of sales on DriveThruRPG. Now, let's assume that WotC marketing offer convinced 1% of D&D Beyond users to try Tome of Beasts V, or about 150,000 sales: or approximately $7.5 Million in revenue. Sure, 15% on 7.5 million would be a lot of money given to WotC, but from a business point of view that's a fair trade.