The easiest way to make enough money is to beat each career event twice. Don't bother wasting money on extra skins till you know you have enough as skins don't advance you to any achievement. Only buy the cars you know you'll use when you actually need them, I.E. if 2 are unlocked for 1 track, just buy the better one. The last car to unlock does so after you beat Champions of champions race and is awarded to you for free, while the 27th-28th cars are opened for the short pikes peak unlimited race on Tier 8, start buying the extra cars after that.
****Taken off achieve360points.com**** Credit goes to the author.