Just speculation. So don't take it as 100% true. Apparently when you open up the store section in the menu, nothing is there but it requests a XML file where you can find "cash cards" along with the collectors edition stuff that range from $100,000 to $1,250,000
And you can find
"Cash is king in this town. Solve your money problems and help get what you want across Los Santos and Blaine County with the purchase of cash packs for Grand Theft Auto Online. All purchased cash is automatically deposited into your character’s bank account. Spend wisely, cash therapy is fleeting."