"Half million" and "You got the message" achievements hints
It's possible to unlock these achievements at the same time. If you have not unlocked the "Half million" achievement via mission money, simply wait until you complete the game. Completing the "You got the message" achievement will add about $400,000 to your bank account. Find all 30 car to then bring cars to the garage for money. Just find SUVs in the neighborhood and sell them for $7,000 when in mint condition. If you follow these steps, you will unlock those two achievements pretty fast.