The reason for the money is to buy business', and Safe Houses.
Don't waste your money on Weapons/Body Armour. There're plenty of FREE stuff floating around, check out the 2012 Weapons Guide.
Go back to Los Santos to kill the street Drug Dealers, too. They drop around $2,000 when killed. Groups of Four Ballas tend to drop a simular amount (run them over.)
The purchase marker for Zero's Shop will be GREEN, when it is available to buy.