Minecraft was cheaper for people who bought it earlier. This is an example, but regardless. Let's say for discussion's sake it's £15 when you buy at release, but it might be cheaper if you bought it during alpha/beta and if you buy it during alpha/beta you get all future updates/DLC's whatever for free as well, as an added incentive to purchase the game. Either way, it's a good model to follow. In this case, I'm not sure if there will ever be paid DLC's, because it seems it goes away from the greatness that was the original mod and I'm fairly sure we'll see a 'One-time only' payment of XYZ and then you will get all future updates for free anyway, from what I gather at Rocket's presentation he did.
