Aggregation sites are an excellent place to get started. This will give you an idea of where the value might be in a specific market at a particular time.
It is good to check the rate directly on the rental company's website. Sometimes, car companies offer you a better deal if you book directly with them instead of an aggregator.
Some websites quote the base price without taxes and fees. Other websites mention the total out-the-door price.
Economy cars are the lowest rate available in a car rental. Unfortunately, they are also the most likely to be completely booked.
Often SUVs are the most expensive option for the same relative specs. For instance, many SUVs might be the same size as a mid-sized sedan or a full-sized vehicle.
The adage, honesty is the best policy, is not commonplace for nothing. It applies in so many areas of life, and personal finance is certainly no exception.