I found the expert free mission go for gold 1 to be the best for grinding out gold. I did contact PhantomSiren (one of the first people to get the full completion of this game), and they recommended the very same mission. It will give you 5900 gold each time. I needed 645k after doing every key, extra, and free mission. I even did the multiplayer achievements before this, and didn't break 400k. Be prepared for a fairly long and tedious grind.
I don't know if there is a way to track how much you've earned, or spent, in game. I waited for the obtain 1m gold achievement to pop, then I spent all my money on health boosts to pop this one.