Hi Leej, the early adopter is considered full payment for Construct 2. There are many more releases to come! We like to release often with small increments rather than big monthly ones. See http://www.scirra.com/construct2/releases for our release history.
Think of it this way, we have Construct 2 and we are selling licenses for it. Because Construct 2 is in Beta we are rewarding people who buy it now as early adopters by giving them a significant discount. Once you buy a license you have a fully licensed copy of Construct 2.
Any new builds of Construct 2 are compatible with your license. You would only need to pay when we release Construct 3 which is far, far off in the future :) We release new builds all the time.