If player got 5 blacksmiths, 5 forts with upgrades and get ability from wizard tower to decreas cost of upgrades, then reaches -107% decrease what makes upgrades for units unupgredable except banners which still costs 13 iirc
If player got 4 sawmills and 5 upgraded forts cost of building wont decrease under 33% anyways (furnaces cost 200, forst 3333 e.t.c)
Reduce of cost for uruks from wizard tower doesnt stack up with this one from furnaces, uruks still cost 400 (only 20% reduce is present)