I think this bonus needs a perspective change. Let me begin at the top.
Britons get their wood cost halved
Bulgarians get their stone cost halved.
If you drop two TC’s in Castle age, grand total, Britons will save 275 wood, enough for a University (to eventually get ballistics and chemistry) and tack on three houses. That’s extremely substantial.
If you drop two TC’s in Castle age, as Bulgarians, they will save you 100 stone, or not enough for a tower. Less than a third of the cost of a Krepost. Less than a sixth of the cost of a Castle.
This. Bonus. Is. Bad. It needs to be expanded or gutted. That’s the list of options. I hope we understand why. The ONLY significant purpose of this bonus is currently to be able to drop a single defensive tower against pressure in feudal and still have the stone to drop a TC in castle age, which loses all meaning if the pressure carries past that tower or if you go to stone to supplement your defenses. It’s a terrible bonus in 95% of cases, which is why I keep saying it, and why it should change.