If you're looking for Portal alone, it is around $20 over STEAM -- to pay-to-download from them.
It took me like 9.5 hours to finish. LOL. Test Number 18 killed me for hours and hours, trying to get it right w/ the timing and all. I was taking my time, but yeah -- probably would've took me 5-7 hours, otherwise!
Some have finished it in like 3.5 hours! So, I dunno -- probably on average, 4 to 6 hours for most to finish Portal, I bet. I really should try the extra "expanded" maps, some time. heh -- they unlock when you beat the game.
About $50 for The Orange Box and the other new content, TF2 and HL2: Ep Two are pretty good stuff.
If you ain't played HL2: Ep One, it's in there -- though, not as good as HL2; not even close. Weakest part of The Box, by far.
HL2's in the box, too -- and if you ain't played that, you really should; it's a very good shooter.
Honestly, you should've probably ordered The Orange Box right from BestBuy.com for $25 (with free shipping) when it was on sale during Thanksgiving Day -- oh, and Black Friday, too. Easily worth that.