I've just been reading the tips for various Ambitions in the Guide, and have one I think is helpful. For the Wealthy Populace Ambition, the thing that helped me the most was to do 2-person quests whenever possible. That way, you'll have two characters getting the benefits from the quest and they can also share items between them. This strategy allowed me to get all the heroes up to the required $7500 and still have 20 quest points remaining, and in spite of having to sacrifice several QP when one of the quests wouldn't recognize a completed step.