In defense of the fee, you likely won't be playing it for more than six months anyways—once you've mastered vim, you probably won't keep playing it.

However, my response really adds more weight to your idea: since we probably won't be playing the game very long anyways, why not just offer it in perpetuum for the same fee? I bet the author will get many more sales with that arrangement, and their expenses won't really change (if someone does become an avid long-term Vim Adventures player, even extensive usage costs mere pennies in server time and tech support).