10. Fix the nut behind the steering wheel
Price: Free at first, then you'll be paying for driving schools
No, your steering wheel (hopefully) isn't about to fall off. The nut behind the wheel is you. That's an old saying amongst drivers, and it doesn't mean you're crazy, just that the single greatest barrier between your car, and your car being fast... is you.
Grab a book on proper driving techniques, search the Internet, or click here, to start learning. It's a beautiful thing when a highly skilled driver in an average sports sedan outright embarrasses someone in a Viper who doesn't know what they're doing.
Improvements don't get much more cost-effective than that.