The findings of this article is the exact premise of AutoSlash.com, a website that tracks the price of car rentals, and even automatically re-books you if and when prices drop. The site launched earlier this year and has saved thousands of customers money in its first few months of operation. In some cases, we've dropped the rate of some folks 5+ times over the course of a month or so. While prices don't drop for everyone, it happens often enough that it's *very* worthwhile to track rates.

FULL DISCLOSURE: I am one of the founders of AutoSlash.com.