First thing you need to know (and probably noticed) is that collecting loads of money in a short time will start a multiplier that can be maxed to 4x which will be shown on the top right of the screen.

Scrollway is probably the easiest and quickest way for you to make it rain because the secret area is right at the beginning and makes it really easy for you to nab the diamond with a 4x multiplier and im gonna show you how.Right at the beginning of the level there are 3 lasers shooting at your direction just get below them and jump in the hole below and the "secret" area will be there,you cant miss it.

Once you're there just dash through the first row, then grab the diamond while the 4x is up and you will net yourself 4000$.
you can grab the rest of the money if you want but the diamond is the real meat of the bones.

And thats about it, just press the restart button and collect it over and over till you can afford some of the best skins in the game :)