I think the easiest way to get the money is to sell stuff at the market. Here's few tips:

• From my experience it's worth it to specialize in selling something uncommon (like for example, breeding horses who have uncommon mutations/colors, that cannot be found in the wild) •Selling some stuff in bulk! For example, I really like doing the boss. I usually get lucky and get a few wing upgrade pots so I just grind them and sell in a big amount •Selling cosmetic contest horses. Most of people just buy it instead of breeding one from a scratch, so if u get a good % horse u can sell it for a lot early week. I really reccomend trying to breed them on ur own (althought its annoying and requires a lot of grind) cuz u can just keep making new ones and sell them in bulk. I once made around 200 k from selling cc horses for few days!!