I just buy a lot of Pearls at Slushland when they're in their lowest price (which can be like 250ish) and sell all of them at the Dragon's homeland (forgot his and his homeland's names) when it's in its highest price (which can be around 1300ish).
And since you can buy like 100 of them, you spend like 25,000ish in total on their lowest price, and sell them like 130,000ish in total on their highest.