It looks like "Sort by Price" would solve this problem. Online shopping of this form has been available for at least 15 years now; surely, it has become ingrained in people to sort by price if you're looking for the lowest price? I always assume that the data that comes back from the search is either unsorted or sorted according to some criteria that makes sense for the merchant.

Another point to consider for rental cars is the quality of the car and the amenities available in it. Sure, by the rental price alone, you could "save" 9 dollars per week by going for the cheapest car, but look at what you would get for that price: a Kia Rio! I would have personally opted for a slightly more expensive option with more creature comforts. An extra 9 dollars per week for the rental is not much when you look at the total cost of the rental, hotel stays, expenses, flights, meals, etc.