I signed up for Steam today which took a long time for various reasons. When I added Portal to the cart it said $9.99. So I have to just download the mac demo to see what it is about.
That sucks, still not too bad for a fun game. I'm enjoying it slowly since I already watched a friend beat it, but if you're looking for bang for your buck, I'd say spend the extra 20 and get Portal 2. Sounds like a longer campaign, plus multiplayer (maybe it's just co-op).