The upgrade vs gold gain ratio does not make any sense. Some examples:
Spend ~100k to gain 100 gold each cycle (1st quest, lvl 32); this means we would need 1000 cycles for return of investment.
Spend ~650k gold to gain ~2500 gold each cycle (8th quest, level 3); needs ~250 cycles for return of investment.
This basically means you can progress further and faster if you do NOT upgrade quests beyond a certain point. Which does not make any sense.
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You'll be able to with the next update!