You are comparing the old simple "Remove Ads" option to Sync Ultra features.
The new "Remove Ads" without additional features costs 14,99 €. Which, yes, is more expensive than before, but that was to be expected. The old price would have been way too cheap for today's pricing situation, especially since he also lost most of his userbase.
That was in response to your comparison with t-shirts.
And yes, scaling does not work in the same way for app development.
A large part of the required work for app development stays the same, regardless of how many actual users there are (excluding server costs (-> Sync Ultra) and probably the amount of support tickets).
But since Sync has way less users now, there has to be more income per user for it to be profitable.
I literally just explained why the price per person needs to be higher now. It is not about server costs. It is about the cost of app development and maintenance.