The easiest way to get this is to go onto the 4th race on the career/story mode and then turn all the assists to the hardest, I was left with a percentage bonus of 69% as I used a manual without clutch! From there simply go through this race over and over as it's not too difficult. It will be challenging at first however once you have done it a few times you'll be getting 1,100,000/1,200,000 every 2 minutes.
TIPS: Go slowly around as the time challenge isn't an issue, there is no need to try and go too quickly, you can also skip a load of pins like I do in the video and still comfortably get 1,100,000 each time!
After an hour or so the achievement will unlock when you have got a career total of 50,000,000 :)