I've never played a game where sandbox mode gave unlimited money. However, there is a way for you to come close. Sign-up as a patreon and get the cheat code (this is actually made for debugging). You can get all the money you want. Although one of the main goals of this game is to make money and lots of it. Otherwise you can edit the starting presets for sandbox mode to give yourself an enormous amount of money. This game is made to enable modding, though it is not complete enough to make that worthwhile at the moment. Don't be passive.