Fortunately for the modding community, Valve decided to run a long with a little mistake which led to the full Source Software Development Kit (SDK) being available for free.
The Source SDK was originally available with the purchase of any Source-based game (such as Half-Life 2 and pOrtal). One of those games is Team Fortress 2 which moved over to the free-to-play-micro transactions-supported model recently.
During that transition, Valve didn't notice that the Source SDK is available with the game and so they didn't remove it from the bundle and it became free for all.
Valve decided not to spoil the fortunate accident. "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 are some dependencies we need to shake out," Team Fortress 2 developer Robin Walker wrote in an email. "But yes, the gist of it is that we're just going to go ahead and make the Source SDK freely available."