Soldato
I would love to know how rich Valve actually is. They aren't a publicly traded company, so all their profit belong to them.
Buy a company report on them http://www.google.co.uk/finance?cid=3784325
Price is what people are willing to pay. It'll be assets tangible and intangible, earnings and forecast earnings which must be quite considerable for them I imagine.
10bn seems conservative enough? EA is half that, but were 20bn 4 years ago. Keep an eye on facebook or groupon even, general tech valuations bare a relation

Or did he cheat at multiplayer or something?