I used Enterprise YEARS ago from Miami to Boston and then a year later for Boston to Miami. Worked out well. I don't remember the cost, but if you could pickup / dropoff at an airport location that cost is a little less I believe.
Also, check out services like Hotwire . com, you don't know the company you use until you book it, but it will give you the prices.
Can easily get quotes for all the big name car rental services online and put together a list... Use hotwire last... I used to book a ton of flights on Hotwire.