All upgrades are free for 12 months after purchase. There is no distinction made between minor and major upgrades. After the 12 month period, you can continue using the version you are using at that time, forever and at no extra or recurring charge, or can purchase a further 12 months of upgrades, which will entitle you to upgrades for an additional 12 months.
As Atomic Scribbler is less than a month old, the future upgrade price is not yet decided, and probably will not be decided for another 10 months. But, as mentioned earlier, you are never obligated to pay any upgrade fee. You can continue to use the latest version released up to 12 months after your purchase without extra cost.