If you somehow run out of gold and you want more of it, there are some areas that I've seen to give you more than others. I would still recommend to complete the Stages from the XP category in order to shoot two rabbits with one shot, but here are some others that seem to give more money than others:

Stage 2-4: If you want to go through something that's really fast and offers a decent amount of money, choose this one. Not extremely rewarding if you're a higher level, but gets the job done

Stage 6-2: For when you get a bit more advanced, complete this stage over and over again for nice amounts of money (and also a bit of XP).

And these would be my farming recommendations for now. Do you have other areas that you found to deliver solid amounts of XP or gold? Share your findings with us in the comment section below.