According to this Rock, Paper, Shotgun report the Source SDK will soon become free to use in its entirety, for everyone. The news comes from Team fortress 2 masterminds Robin Walker, who credits the upcoming release to the change in Team Fortress 2 to a free-to-play game.
The Source Software Development Kit contains all the tools you'd need to create a modification for a Source game including the Valve Hammer Editor, Face Posers, Model Viewers and more. The only way to gain access to the SDK right now is to purchase a Source-based game. The news came from a response Walker made to a Reddit thread about Zombie Panic Source and No More Room In Hell. Readers asked Team Fortress 2 developer Robin Walker if buying an item in the Mann Co. Store would give access to the Source SDK, He said:
"Yep. That said, your email has triggered a process here that made us re-examine that, and we’re going to just go ahead and make the Source SDK freely available. Thanks for making us better!"
RPS later emailed Walker and got this response:
"We are in the process of getting it all done. It’s a bit messy because we have multiple versions of the SDK, and there’s some dependencies we need to shake out. But yes, the gist of it is that we’re just going to go ahead and make the Source SDK freely available."
Expect an announcement about the SDK release sooner rather than later.