Why do these have such high upgrade costs for lvl4/5?
For other production buildings, you gain from the buffs working better as they upgrade.
Storage house is the only other upgrade building with no production and its both cheaper and the scale or storage increases as you upgrade (100/500/1500/3000/6000).
I think the upgrade price should be matched with storage houses (500gc/300marble for lvl4 and 500ewp/300granite for lvl5)
and you should get +20 for lvl4 and +30 for lvl5, so in the end they would give 30/40/50/70/100.
At the moment a noble deed bought through trade probably costs 700gc+ cheaper than the lvl4 upgrade, doesn't use a license and provides 40 extra population, a bit of a no-brainer which is the better option unless you find yourself with no island space left at all.