Their page has (roughly) 300k followers. That's $100 per reachable user per year, which is insane.
If they were using that additional $10mm to get the 300k followers to begin with, that's an even worse indictment of FB advertising for traditional brands.
Remember that they don't have just one brand on Facebook. For example, they have 1.3 million followers on their Chevrolet page. They have 1.1 million for Cadillac. 400k for Buick. 660k for GMC. 525k for OnStar. 350k for their "TeamChevy" Chevrolet racing page. Now, there's sure to be some overlap there; but that's still easily 2 or 3 million followers.
That's a factor of 10 more than you were estimating; and that's just based on a quick poll of brands that I took a look at.
If they were using that additional $10mm to get the 300k followers to begin with, that's an even worse indictment of FB advertising for traditional brands.