http://www.vgchartz.com/article/85690/portal-2-day-one-sales-report-americas/

As you can see from the link above, Portal 2 sold more than 600 thousand units in one day only in America. This is more than Crysis 2 sold in the first week. So you could extrapolate that they could sell 7 times more than Crytek if that stays the same way.

Portal 1 sold 4 Mio retail units. There are probably 2-3 Mio additionally Steam units. Crysis 1 sold 3 Mio. So Valve probably sold at least double the amount.

I find this really interesting. I mean this is a pure puzzle game, so it serves some kind of a niche market. And this niche is obviously quite big.

We saw another success lately in another niche: Minecraft.

Or look at the huge amount of sold units of "Angry Birds".

It looks like it is more rewarding to serve a niche very well than following the trends of big publishers.