If I'm not mistaken, you can download source SDK for free when you have HL2 or something.
Just depends on HL2

M, which I would recommend to get anyway, because you can play most of the mods, which will guarantee you more then 300 hours of satisfying gameplay.
I bought my HL:2 when it was released in holland, I thought it was something like 2005-2006 "not so sure anymore" and I still play some of the SourceMods!!!