Just... Don't even try legit. If you enjoy driving in a straight line dodging traffic for hours upon hours on end, sure, go for it, but its going to take a really long time.
After I got the 1000 runs achievement, I only had about 200k saved up. I didn't want to do another 4000 runs, so I wound up using Autohotkey to just hold down my W key. You could do this manually by taking a heavy object such as a hammer and puting it on top of your W or even your up arrow key. The mode: Bomb on Peaceful difficulty. There is no traffic, and you just keep driving. Go to bed, go to work, come home and maybe go to bed again, that's how long this takes (though you can check your progress as you go) and maybe then you'll have over a million. I did it over two nights of sleep. I got about 500k a night, my final tally being 1.2 mil. To collect your winnings, all you have to do is stop holding w or up and the car explodes giving you a bunch of money.