The best way to earn money in Gangstar Rio is undoubtedly to do the missions, while not spending too much on cars and unnecessary extras like that.
You can also do Taxi and delivery missions by stealing one of those vehicles and tapping the radio icon at the top of the screen. These don't earn you that much money, however.
Another method is to get the police on you and kill as many of them as you can, while collecting all their weapons and money. You should be able to make a profit by doing this, even when counting the money you have to spend on medkits etc. Try to pick a spot near a police shield so that you can easily stop the police from chasing you once you want to. Killing people in suits, in particular, is a good idea - as they drop the most money.
There are various heist missions you can perform, indicated on the map by a pair of arrows. Robbing the bank and hijacking the drug deal will allow you to net a lot of money, but once those have run out you will begin to struggle for it. Also, there is no way to save the game once you complete all the missions, so it becomes less and less worth getting all the expensive vehicles.
Remember, you can also buy money using the in game store, should you have the money for it.