Not sure if this has been posted already,
I found this whilst attempting a different money glitch.

This best works if you have an adder for maximum cash,
Also need to own a stretch limo,

"MAKE SURE ANY PERSONAL VEHICLE IS IN GARAGE"

Right,
Start on story mode,
Open menu and start a "closed friend session"
Once in there, invite 1 friend.

Phone Pegasus and order your stretch,
Go collect it from it's location,
Drive it to your garage "preferably 400k garage"
Park it on the left side of garage door so when you get out, the door can't open.
Jump in your most expensive car and drive out,
Because the door is unable to open, you will spawn down the road.

When spawned, phone "mors mutual"
If it worked, it should let you buy the vehicle back like if it was destroyed,
If it does..... It will say in top left corner, you will find your replacement car in your garage,

This is where your friend comes in,
Get your m8 to sit in the car you have outside already whilst you go in and retrieve the same car from your garage,
Both drive the cars to the customs garage and you sell your car for the large sum, then get your friend to give your car back, you then drive in to customs and re-insure that car,
This can be done over and over again.

REMEMBER TO REINSURE THE CAR OR IF IT IS BLOWN UP, you will lose it.

Hope it helps,

Also...... This is running on 1.11