Best Answer
Valve isn't a company you can predict. To some degree every publisher is always out to get more money but with the money they make off STEAM they can afford to be more laid back about how they approach game development.
But when you consider the fact Gabe Newell the head of Valve's game development makes games by design first and monitization a distant second, the only real answer to this is if he thinks a third portal game is will deliver on the design front yes you will see a new title, if not they can always make more portal 2 DLC and still keep portal in front of consumers.
BTW the way Gabe Newell thinks isn't my opinion he has come out and said this himself.
http://www.joystiq.com/2011/08/30/gabe-newell-on-monetization-before-game-design-spoiler-it-stin/