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Money can be used for many things throughout Stilwater. Buying weapons, tricking out cars, buying clothes and food. There are many ways to get money in Stilwater, they are:

Action	Description
Complete storyline missions	Every time you complete a mission, you get cash, more of it depending on how far along you are in that story.
Activities	Completing activities not only gives you respect, it also gives you money. The harder the activity was to complete, the more money and respect you'll get.
Robbery	You can stick up a store, or you can go in the back and crack the safe. You can also blow up a Titan Security Van and steal the money that comes out (which can be up to $10,000).
Hostage	If you hijack a car, from the driver's side, that has two or more people in it, you can choose to keep the other person(s) hostage. To do this click Y when you get into the car. You have to drive at high speeds, so they can't get out, for a certain amount of time.
Loan	You can take out a $50,000 loan in the Downtown District. That means if you really need that RPG Launcher for a mission, go take out the loan, beat the mission than pay it back. If you don't pay it back within the time limit the lenders will come after you.