How To Unlock: Spend $6,000,000 in Fortune City.
After starting the single player game, make a save, quit and start playing some multiplayer. Money gained in multiplayer can be transferred to an existing single player save file, so while you're getting your multiplayer achievements, you'll be working this achievement aswell. Chances are you're not completely done with this after your multiplayer madness, so I'll list the quickest and unfortunately also the most boring method. Research your gambling skills by gathering all 3 magazines:
Ragazines in Royal Flush Plaza
Stan's Large Print Books and Magazines in Pallisades Mall 2nd floor
Shamrock Casino on the Silver Strip behind the bar
Keep your 3 magazines in your inventory and make your way to the big pink slot machine in Slot Ranch Casino. Push the 1000$ button, wack a zombie while you wait, and you should be raking in money fairly fast. Once you have your 6 Million, be sure to spend some cash on various items around the shops. Things you need to buy/do for other achievements:
- Buy SUV Keys for 2 million at the Silver Strip pawnshop
Spend approx. 1 million at the fortune teller at the Silver Strip to obtain a hidden combo card
- You can spend the rest as you see fit.