DiRT Showdown

For this you need to earn a total of $1,000,000 both online and offline.

Offline in the Showdown Tour winning in the first tier gives you $5,000 per event, second tier $10,000 per event, third tier gives you $20,000 per event and the fourth tier gives you $30,000 per event.

Completing challenges earns you $1,000 and winning finals gives you an extra bonus.
Completing the Showdown Tour will yield you a few thousand short but you will be close.
Note: Redoing events you have achieved 1st in will not yield anymore prize money.

Online races do not yield much money but still count.

You do not need to have $1,000,000 total only earn, you can check your progress via Options & Extras - Extras - Statistics - 'Cash Earned'

Just as Spilner said, once you complete the entire Showdown Tour you will still be a few thousand short, even if you took first in every event. I did not have a VIP pass for the game because I rented it, so the only other way to make money is to win challenges against friends. I was $20,000 short so I sent 20 challenges to my silver account on my xbox. Then I signed in with the silver account, accepted a challenge and quit out right away forfeiting the challenge. I did that for all challenges, then signed back in with my main account. I went to the challenge menu and it awarded me $1,000 for each challenge I won, giving me the $20,000 I needed and the achievement popped. This method also gave me the "Running the Show" achievement as a bonus for dominating a friend in challenge mode.

Recently went through the game and like a lot of people was 10-20k short of the million.

If you have a second account, send challenges from your main account to your second account. Go on your second account , accept and quit/forfeit each challenge granting your main account the win. (Same as you would have done for the challenge related achievements). For every challenge you ''complete'' earns you $1,000.

I suggest using the Hard Target event in All Star as you can be destroyed in 30-40 seconds by boosting yourself into a wall. Send the challenge, restart the event as many times as needed.

I got this achievement at the end of the "Showdown Tour", just as I finished the final race along with my Gold, Silver and Bronze etc. Basically all I did was to ensure that I came in first in every race, and had Gold in everyone therefore.

Then came the vital difference from what most have mentioned...you are $20,000 short of the million at the end...but if you sign up to Racenet as it suggests at the start of the game...you get a $20,000 bonus, voila...dead on a million at the end of the tour, no sweat :)