Originally Posted by Opalia24

I calculate with an upgrade cost of 4000 coins per unit.

This right here is a big problem with your calculations. The coin cost per fodder used is the same as the knight's level times 100. This means that once you hit level 40, then sure your upgrade cost will be at 4000 coins per fodder, but once you're above that it will be a lot more. The exp required is a LOT more too. It took me about 6-7 level 100 fodders to get a girl from 50 to 60 and at that point the cost is 5k coins per fodder.

Imagine trying to do that with level 5 and level 20 fodders, which do not even grant half of what level 100 fodders give. Heck, imagine trying to level a 5* knight to 70 with just level 20 fodders. You'll get about 5 levels at most each time you feed a full 10 of them above 50 and each time is going to cost you 50k+ coins. Meanwhile 10x level 100 fodders will cost about 50k and get you way higher.

Sure, if you only have bronzies lowbie fodders might still be a thing, but once you start getting a fair deal of gold units (either through gacha or events) the costs will pile up somewhat insanely much.