Collecting the treasure at the end of each level after defeating the boss will give you $1,000,000. There are well over $1,000,000 in collectable gems throughout each level, so scour the levels to find hidden gems that appear when you jump through them, and keep an eye out for secret rooms. You also get a bonus modifier at the end of the stage based upon your difficulty. If you find at least a million in gems in each level and get the million for beating the stage, you should get this trophy after beating all five stages. Maybe even four if you find enough/are on a high enough difficulty. If not, Mount Vesuvius (the final stage) has gems in it too. You can also replay levels after finishing the game.
Just remember that you only keep the money you find if you finish the level. Exiting the level or getting a game over will not net you any money.