Ah, you're not reducing the cost, you're increasing the purchasing power (resource value).
So the diminishing returns when stacking these bonuses is by design.
I hope they will adjust the tooltips for these bonuses, as the currently displayed information is just wrong.
At least change the values of the shown cost reduction to the actual price cut. However, it would probably be better if this purchasing power mechanic were properly explained in a tutorial and the tooltips used less misleading phrasing.
