I don't worry about the money. I've bought maybe 2 cars, and have a ton in the garage that I won. I can usually find something I already own to win. And that, makes a ton of cash if the difficulty and rarity is high enough.
I just wish you could sell the cars you won that you don't want for more than $100. :/ I've got a Honda Civic I won in a race, put maybe 30k into it, and I used it to smoke the 250 and 300 hp series races with 247Hp.
Tires are your friend... And weight reduction, brakes and transmissions. You can make a 100HP car move pretty good doing that stuff.