The vehicles purchase price have been amended due to a recent patch however this does not involve the "Armored Kuruma" vehicle which remains at the same price so the method used does not change.
If you already have 3.4 million dollars then you can use the method below:
•Buy the Armored Kuruma for 525.000 dollars.
•Sell it immediately for 315.000 dollars.
•To reach 8 million using this method you'll have to buy the Kuruma 15 times plus 125.000 dollars,this way you'll have a net loss of roughly 3.400.000 dollars.
•Once you bought the 15th Kuruma you'll have spent 7,875,000 dollars,simply buy a few more vehicles for the remaining 125,000 (three "Dinka Enduro" sold at Southern San Andreas Super Autos will be enough) to go over 8 million to unlock the achievement.
This is the smallest amount of money you can lose as of right now.
•You can keep buying and selling the same vehicle to reach 8 million.
•The achievement unlocks as soon as you spend the required amount.
Tip to speed up the Buying/Selling process:
Buy the garage called "Unit 76 Greenwhich Parkway" for around 120.000 dollars which is located near the airport,the Los Santos Customs is very close to it,take the short cut by destroying the fences on the side and you can sell 2 cars per minute.
Vehicles included in the Heist DLC:
These can be purchased trough the Internet option on your cell phone.
-Dinka Enduro (Bike) - Available immediately
-Vapid Guardian (Truck) - Available immediately
-Karin Kuruma and Kuruma Armoured (Sports Car) - Complete the "Fleeca Job"
-Lampadati Casco (Sports Car) - Complete "Prison Break-Station"
-Principe Lectro (Bike) - Complete "The Pacific Standard Job"
-Mammoth Hydra (VTOL Jet) - Complete the "Humane Labs Raid-EMP"
-Buckingham Valkyrie (Helicopter) - Complete the "Humane Labs Raid"
-HVY Insurgent (Armed and unarmed variants) (Truck) - Complete the "Humane Labs Raid"
-Karin Technical (Truck) - Complete "Series A Funding"