The Tech Report writes Valve plans to make its Source Engine SDK available for free:
RPS points to this Reddit thread, which says a mod playtester by the name of Riley asked Valve if grabbing the free version of TF2 and buying an item from the Mann Co. store would grant him access to the Source SDK. Valve's Robin Walker reportedly shot back, "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!" When asked for confirmation by RPS, Walker added:
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.