My $150
scangauge paid for itself in about 7 months. I only commute 11 miles each way, and my car is already fairly efficient. Any more distance, or a thirstier car, or higher gas prices, and the break-even point will be sooner. Highly recommended.
@boxchain - we basically have the same car and a similar commute. You can get great numbers if you're willing to push for it.