Valve is a private company, so we don't know for sure how much they are worth, but if MS wanted to buy them, I'd wager my house that the figure would be way more then they spent/will spend on Activison. By a magnitude of 3 or 4 at least. Valve is a literal money printing machine. They make so much money that they don't care that they have a squeal they could release that would make more money than damn near any other game ever released.

And for the record, I'm not saying Microsoft can't afford them, just saying it would hurt to do so.