It's a one time payment for lifetime subscription. Problem is, they keep adding in new packages (because D&D always changes) and the packages can range anywhere from $4 to $20 depending on the size or importance of it. For instance, the APG (Advanced Player's Guide) and Core manual are very expensive compared to the other packages which can be Ultimate Armory, Adventure Paths, etc.
Herolab, IMHO, is the best thing I've gotten for RPGs because it takes the mass amount of time consumption that is done researching through the rule books.