There's a promotion going on right now - the basic version for $5. If you buy it, you will get access to all updates costing 5$ or less. When the promotion ends, the game will cost 10$ again and so it will be until the end (maybe after the end we will reduce the price, as support will no longer be relevant). If you purchase for 10$, it's a one-time fee for all future updates until the end.