Probably the last achievement you'll earn in the game. When I finished all the other events, I still needed another $2.5 million. I found the quickest way to grind it out was to go to World Series tier, and replay the second head-to-head race (Notting Forest Challenge). It takes just over a minute, and you earn $40k every race. You can replay either of the last 2 championship races for $250k-$300k if you get bored, but they take much longer and the return per minute is nowhere near as good.
Note: while you don't need to do the Grand Finale tier of races to get the career complete achievement, you'll need to do them to earn the final car. You'll be gifted a McLaren after completing the "Super Secret" League Of Legends races, which appear after completing the Ultimate World Championship.