I wasn't considering cover blurbs at all. That's a different story, though also something that has more effect on some people than others, and perhaps not as useful as Tor thinks (the LOVE their cover blurbs). However, I have indeed heard of a lot of people who picked up Brandon for that reason. But then, it turns out that Scott Card is effusive whenever he blurbs a book, and not all of those books go on to success.
And whether sales are definied by how much $$ you put into marketing is HUGE question mark. Now, if some complete nobody first-time author got the embarrassingly huge marketing push that Brandon is getting on WoK, would it make a sales difference? You bet; it would make an enormous sales difference even if the book is only OK. But what percentage difference is it going to make for WoK? Much, much less difference than it would make for a no-name. A lot of the people who are doing things like entering the ARC contests would have bought the book anyway because they're already Brandon fans. I do think the marketing will make a difference, but this is something you can't scientifically test. You'd need an alternate universe in which little to no marketing was done in order to see what difference the marketing made. Of course if the book does sell well, the marketing department will pat itself on the back for a job well done, whether it sold well because of anything they did or just because people read THE GATHERING STORM and liked it and decided to pick up Brandon's next book. Which is probably going to be the greatest single contributing factor to its sales.