I want to know if it could be possible to multiply % bonus instead of adding them to prevent a bug like in the situation below:

Let say you have a building that reduces gold cost of buildings by 50%. If you build 3 of these buildings, you cost is reduced by 150% which means that everything you build is free.

Now my suggestion is that bonus could be multiplied together instead of being added. This is useful when the bonus reduces a value instead of increasing it. By multiplying percentage, the situation above would give:

The first building reduce by 50%

The second building reduce by 50% of what is left which is 50%. So it reduce by 25%. For a total of 75%

The third building reduce by 50% of what is left which is 25%. So it reduve by 12.5%. For a total of 87.5%

This way, my bonus will never reach above 100% which is a good thing when it's a bonus that reduce a value instead of increasing a value. It creates some sort of logarithmic curve.