I think we are fine, with all the canal building we are doing general economic output is on the rise, though population of trained wizards much less elven Archmages and Von Tarnus... er plural is unlikely to go up anywhere near as fast. Not to mention that the more of them we raise the more wealth there will be to raise more of them.
The fact that the pot is growing doesn't mean it isn't a problem, as the real issue isn't the cost, it's the opportunity cost.
Just making some numbers up for illustrative purposes.
Let's say the Empire has an infrastructure budget of 100.
Waystone Design A costs 5 and any plausible number can be made per year
Waystone Design B costs 1 and up to ten can be made a year because of skill constraints
Building a castle costs 10 and any plausible number can be made a year
Assume Waystone A & B are functionally identical
The Empire has to choose which it prefers. It can:
Build 20 Waystone As and 0 castles
...
Build 14 Waystone Bs and 3 castles
...
Build 10 Waystone As and 5 castles
Build 10 Waystone Bs and 9 castles
Build 0 Waystones and 10 castles
Clearly, if you want some castles, under a scenario where you have a limited budget dpending on how much you relatively value castles and Waystones, at some point it's better to select Waystone B than A, as the opportunity cost in turns of castles not built is much lower with Waystone B. Even as economic output increases there are points where that remains the same, depending on how important castles are. There's a trade off between castles and Waystones until the budget grows to be enormous or the cost of Waystones shrinks to be insignificant in terms of the overall budget.