If you're patient... watch the
Steam website for special deals that come around.
I bought my son another copy of the
Orange Box SW (as his Xmas present, in advance!) back in August for just $40 (~$10 off) as an "End-of-Summer" deal just to get Portal, and for the additional $20 you get all those other (HL) games, too! (We got them to make HL(2)-based LAN gaming here a bit easier.)