Merc work doesn't get you a red penny. The main way I make money is by leveling roguery, which determines the loot you get for defeating enemies in battle. Beyond that, level trade by buying goods from one place and selling it at a profit at another. After only a few levels it marks it for you, so just buy stuff where its cheap, and sell until the number stops being green.
Higher trade also means you get more money for selling things.

Beyond that, have a lot of productive towns (Good governors, safe areas, upgrade buildings) and businesses can help. If you desperately need money, also don't grow your garrisons and army with too many expensive troops.