Rocket/Dean has said on Reddit he's hoping for free content updates along the lines of how content was added freely to Minecraft. Specifically just this week, he also said he hoped the Standalone sells so well that literally everything will be free for the lifetime of the game (I took this to mean heavy development stuff, like the underground shelters, new maps, etc.).
We'll see, either way I can't wait to give this a go. It'll be worth the price of admission.