I am afraid to say that this achievement is hard to get, however one thing that the other solutions don't mention is a cheat to get unlimited boost.
This cheat is Left, Up, Left, Down, Left, Down, Right, X (using the D-pad)
N.B. you must enter this and any other cheats in the menu.
Before you ask, no, this has no effect on any of the achievements meaning it will unlock anyway, so too does the reward card.
Essentially this walkthrough will assume that you are using the unlimited nitrous cheat. I would start the career with the muscle car, play through winning races, using Samson to get an extra $200 every win (every little help). DO NOT BUY ANY UPGRADES EXCEPT NITROUS.
As soon as possible buy nitrous and upgrade when possible. When tuning the nitrous, fill the bar on the velocity side (if using the cheat), this will increase speeds dramatically, and of course will last for as long as you hold the boost button.
Win races until you unlock the boss race, now save before you start the boss battle, you will see why in a second.
Win this and you will get the option of picking 2 of 4 secret cards; 1 of which will be the pink slip to the bosses car. If you do not get this pink slip, i would recommend you immediately reset your xbox and restart from where you left off, however this is not always necessary, it is possible to win the next area of the nfs world with the first car, as long as you are using the cheat.
If however you do get the pink slip, i would recommend using Angie's car (with the nitrous cheat) as it is faster. Continue to use the pink slip cars that you unlock from other bosses too.
After you have beaten Angie, Simon will become available to use as a wingman and he adds 10% to your total winnings. Now also sell your old car for extra cash.
The rest is simple, just keep playing through using the bosses cars (try to get Kenji's and Wolf's pink slip as well). Just keep using the nitrous cheat and you should get it about half way through the stacked deck region (last 1/4 of the game).
Any cars you unlock and subsequently don't use as you have a better one, sell for extra cash.
some other useful cheats include:
infinite speedbreaker: Down, Right, Right, Left, Right, Up, Down, X
infinite Crew Charge: Down, Up, Up, Right, Left, Left, Right, X
earn an extra $10,000 in career: Down, Up, Left, Down, Right, Up, X, B (can only be used once)
This achievement is easier than most people say, if you use the cheat, and remember only to buy nitrous, that should be the only thing that you buy through out the whole career.
As soon as you get your $1,000,000, in celebration you should buy your favorite car (mine being the corvette Z06) and upgrade it to the max.
Happy hunting ;)