This is footage of me playing the Cone Crazy side-mission in Vice City, known for being very exploitable. You must pass through 5 coronas as quickly as possible while being timed. The key to exploiting this mission lies in the large amount of extra time left over, since it's a very easy side mission. This mission pays out $200 when first completed, and twice the amount every time the record is beaten. Beating the record for the 23rd time rewards a payout of $838,860,800, for a grand total of $1,677,721,600. Refer to the payout chart at the end of the description.
The mission adds 12 seconds to the timer each time a corona is picked up. In the PC version, 1 second is subtracted each time the player completes the mission, until it reaches a minimum of 6 seconds per corona. On the PS2 version, passing through a corona will always add 12 seconds, making it very easy to beat 23 times in a row to receive the maximum payout. Keeping track of how many seconds you had left over is key to grinding money with this mission. Keep track of this number on paper.
If you receive the reward for completing this mission more than 23 times, the payout will be a large negative number, and you will be left with no money. I exceeded 23 attempts on the PS2 version to find out that after a few more tries the payout will become positive again, but it's not possible to achieve this on the PC version, you will be bankrupt in the meantime, and the reward for 23 attempts is more than enough.
One important thing I left out of the video was how to trigger this mission multiple times without saving/restarting the game. Simply park the Stallion back where you found it, get out, and get back in. It does not matter what direction the car faces as long as it's mostly on top of the invisible mission trigger.
Save File (1% completion, max money): https://www.mediafire.com/?1sarzuu234ytugs