So we have a game which relies on three resources. Gold coins, Runes and food.
Considering that both runes and food show a decay rate, I wondered why gold doesnt do you. I mean your vaults can only hold so much right? At some point it just gets easier to be stolen (by random thieves roaming your own lands).
My suggestion is thus to introduce a gold decay. Numbers are up for tweaking.
1000 gold coins/acre are automatically safeguarded, going over that 10% of that amount over 1k gc/acre gets robbed every hour.
So it would be
Decay: (Actual gc - acres*1000)*0,1 (with a nice if condition of course to not generate gc instead of decaying it).
Hourly update: Current gc+GC generated - gold decay
COnsequences: provs of 400 acres can stash 400k gc, provs of 2k acres 2 mil, 10k acres 10 mil. Over that you will loose part of what you have.
(now Top KDs go ahead and flame it cuz it ruins your current pump methods :D)