Most every American enthusiast knows about the $42,000 Mustang Shelby GT500. Its supercharged V-8 thumps out 500 horsepower. However, today there are dozens of equally powerful cars, so 500 ponies isn't much to brag about. A trip to Carroll Shelby's Las Vegas workshop with your GT500 and $28,000 gets you a 100 horsepower boost. Congratulations, for $70,000, you've discovered the cheapest way into the 600 Horsepower Club.