A lot depends on luck and which way your girls go during the Gold Rush mission. The longer it takes them to reach the goal the more gold they will collect. I just did the 80 Stamina mission (to get the 2 Flower Gems) and got a little less than 20000 Gold from it.

I managed to nab every coin from the 80 stamina mission though, and that yielded something a bit above 20k. It's basically what i used to calculate efficiency. The best possible from the 80 stamina vs whatever i got from the 4-3 + garden runs.

Currently waiting for more stamina and it will take about the time for a gather seed to be ready which is 2 hours. So far i have earned 29738 gold and i expect that to be above 30k once i finish in 2 hours and level up. So that'd be 30k+ in 2 hours, twice, as opposed to the 20-21k every 3-4 hours from a gold daily mission.

What you say is true though, depends on luck. I did say i got 13k from that gold mission once, that was horrible...