Getting this takes quite a bit a cash, and Dirt has a bit of a gotcha built-in. Once you win a race at any given difficulty level, the prize money drops for that level and all the ones below. So, If you do all the races at Amateur, you'll lose most of the money that could be won at Rookie and Clubman.
Working up through the difficulties on each race maximises your cash generating potential, this also gives you plenty of mileage towards the 1000 miles driven achieve.
Also, don't be afraid to have a crack at Pro Amateur and Professional on the Crossover races, you can get $75,000 for coming third and one of the guys is normally really slow, even on Pro. Again, once you have won a prize, the money drops. Even the second and third place prizes.