Steam only costs money if you _want_ to be a subscriber (Platinum Pack) and get future new game releases for no additional charge.
ie: You by HL2 and a steam subscription. An HL2 expansion comes out, you get to download it for no extra fee. TF2 comes out (riiiiight lol), same thing. CS2 comes out, same thing. Everyone else will have to buy them for the regular price just like any other game.
Other than that, it is free.. always. You dont have to pay for steam to play the games, download patches, etc etc, ever.
Dont know how much the so called "Platinum Pack" will cost tho.