You have victory conditions for science, culture and faith yields, so why not for gold, i.e. economy? It could be something like having gpt higher than all others combined - or over 50% of all others combined if 101% is too much. Or it could be depositing gold throughout the game towards a certain target that could be fixed or fluctuating. You "fill the bank", you win an economic victory. I think the game's missing a trick by not having an economic victory.