I actually know, so I will answer.
Brandon's contract is with Harriet. He gets an advance from her that is multiple times larger than what Tor gave him as an advance for a single one of his own books in the past. When the contract was renegotiated he gets a bit more for each of books 13 and 14 than he got for book 12. He does get a royalty but it's a percentage of what Harriet gets, not a percentage of the retail price, and it's subject to the advance. US sales count much more than international sales.
The advance for Way of Kings was not as high as the advance for TGS but it was still very high. If Way of Kings starts selling enough for him to earn royalties, and remains popular, he will end up making much more from the Stormlight Archive than from the Wheel of Time even if it sells only half as much. Ancillary rights are also much much better. And he'll get a huge bonus if a book in the Stormlight Archive hits #1 on the New York Times list.