There are two games that have been found to gain money easier: Blood money and Hotwire. In Hotwire you gain $50 for every vehicle objective you capture. While Hotwired, you earn an additional $15 for every second that you have the vehicle captured..DO NOT SLOW DOWN! The higher amount of tickets in a server, the better.
You can also gain for this achievement by selling various items you earn from Battlepacks, for example, if you have 10 200% boosts, sell them, Paint jobs can also earn you $5,000 and more. Gather your friends and have a marathon to get that sweet $1,000,000.