To make infinite money, you need to have at least 130 gems (but preferably 260, or even better, 390). Then all you have to do is go to the item shop in Scuttle Town, and purchase the three-pack of Monster Milk (NOT the individual ones sold for 50!) for 130 gems. Once you have that, exit the item shop and head all the way left to the sobbing Town Mayor - who just happens to have an endless thirst for Monster Milk, and will happily pay you 50 gems for each one you give him. Since you bought the three-pack for less than the price of three individual Monster Milks, giving him all of your Monster Milk nets you a profit of 20 gems per 3 Monster Milks sold - meaning that once you can buy 9 Monster Milks, you'll be making 60 gems profit every single time you do it, quickly racking up the cash.