How to get cash is really simple, you visit this link [HERE]
And exchange gold to cash with a 1gold:2cash ratio.
The higher level the boss the more gold you get, the only exception to this is Cherry Blossom Sakura.
Dungeons
These dungeons can also give you cash upon clearing it. However it seems to be really low amount of cash compared to getting gold and buying cash.
And exchange gold to cash with a 1gold:2cash ratio.
Easiest way to get GOLD
The easiest way seems to be to go to Edge of the World(Lv95+) which is the highest Level normal Mission Mode.The Boss will drop 1000g each coin you pick up. Note, only the boss will.The higher level the boss the more gold you get, the only exception to this is Cherry Blossom Sakura.
Dungeons
These dungeons can also give you cash upon clearing it. However it seems to be really low amount of cash compared to getting gold and buying cash.