Hint - Waste Not Want Not
It is possible to get enough money to purchase all upgrades but don't buy everything. Some upgrades are replaced by better upgrades, so you should only buy the ones that you are going to use. For example, there might be three levels of silencers but you may not want to waste money on the first and second upgrades, because you can only use the most powerful silencer.
Hint - Blasty, Blasty
Once you've a few more dollars and decided to upgrade your Silver Ballers for more killing power, try using long slides, magnum ammo, full auto, and take two Silver Ballers. Enjoy mayhem.