I recommend to farm this achievement after completing the game, because the last race of the game is very easy and will net you the most money in the game.
One race is taking about 2 minutes and will credit you 50.000. The last track has a lot of sharp turns so I have taken Horns Hot Rod, because this car has a great drifting ability. So I have had nearly the whole race a full nitro.
I am not good at racing games. Despite of that the second placed driver was very far behind me.
Look at the video for more details.