I did some mats on this, and the gold cost is just insane for the upgrades. I converted the gold to marble and hardwood, without buffs (wich really makes no difference for the results, if you get 1.5x (semi-realistic 2x buffing with 6h buffs if you have a job to go to)). I ignored the tool cost wich is high aswell.
Storehouse
Level Hardwood Marble
1 120 100
2 300 150
3 18667 18667+300
So from level 1 to 2 it costs 250% hardword and 150% marble to upgrade, but from level 2 to 3 the same is 6222% for marble.
I havent upgraded any buildings to level 4 so i cant make the calculations for this as this costs really are a game breaker for me.
Maths:
One gold mine that will give you 37,5 gold over 16,25h costs 1400 hardwood and 1400 marble (and 1500 tools)
Cost to get the 500 gold for upgrading from level 3 to 4 (300 ore per mine, 8 ore per coin, 1400 hardwood and 1400 marble) =
(500 / (300 / 8)) * 1 400 = 18 666.6667