From what I've read in the forums the best way to make quick and easy money is to do the second special condition event and sell the RSC Rally Car you win. With race money and selling the car you make around $265k. But if you really want to make some big mulla ($900k+) you can do that special condition race twice, sell both RSC Rally cars, buy an Audi TT Touring car, give it the stage 4 turbo and enter it in European Events/German Touring Car Cup (its written in german though) in B-Spec mode. Set speed to 5 with the overtake option, put the time scaling on 3x and sit back and relax for the 5 races of that event. You'll get a ton of cash from winning the races and the series and the CLK GTR Race Car can be sold for around 750k on top of that. Just make sure you drive the TT a bit first and give it a good setup and you're good to go. I now use that race to make money and it only takes me about 15-20mins to complete in B-Spec mode.
EDIT: And yeah if you hit CLEAR it wont erase the prize car from your garage if you have it, you'd just win a second one, and a third one and so forth. Only thing you loose are the trophies for the event and the % but that is all re-gained once you finish the races again. Cheers!!