It's really, really hard to say no to $10,000,000. Sneds may have made $5mil this year, but that's half a career's worth of cash for most guys that see the tour. To turn away double that amount in a day, based on a potential for "legacy" seems pretty iffy.
So, how much is the green jacket worth? Let's say that Snedeker is on par with Mike Weir: he's had a phenomenal year, he'll have an above average year next year, and then maybe fade into 115th in the world obscurity. (Just for argument sake--I'm not commenting on Sned's potential.) So how much is that green jacket going to be worth to that golfer? He'll get a spike in endorsements for a couple of years, and some appearance fees. He'll get to play in the Grand Slam, which will be worth a few hundred thousand. He'll get to come back to the Masters for the next 25 years, probably make the cut (in a weak field) 2/3 of the time, and cash at least one decent paycheck every year that he wouldn't have access to without the green jacket.