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:
All trademarks are properties of their respective owners.
News CGI copyright © 1999-2015 James "furn" Furness &
All rights reserved.
Chatbear v1.4.0/blue++: Page generated 31 August 2015, 20:14.