There are several methods of obtaining money in GA2. The first and foremost is just to play matches. Play matches nonstop, and do your best to kill as many people as possible. It sounds simple, but it really is the best way to earn money. Attempt to earn the rewards from fighting, such as Murderer, in order to gain more WIN for your fight. You can also take on missions, or sell the items you earn from fighting to the market. and, of course, you can use C to charge the WIN on your account.
Whenever you rank up, it's pay day(not unless General Daigokuin Raizo gives you a quest or something). The money you gain depends on the Rank you "ranked up" to. Everytime you rank up, don't forget to go to the General's Office for reward money and some items(weapons, synthetic materials, and Lucky Coins you can use at Mora Shrine)!