A reddit thread has an email from Valve's Robin Walker confirming theories that Team Fortress 2 going free-to-play means the Source engine SDK will now be free to all as well. There's confirmation of this from Walker on Rock, Paper, Shotgun:

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.