I have rented from the airport and it was cheap. Even dropped it off at the Hilton in DTD. I was arriving by plane from a business trip 8 hours before DW/DD/DS.
Check the web for the best prices. All of the companies are trying to attract clients. Use any affiliate discounts you might have (work, credit card, organization). Book a pick up that is convienient and go from there. There are many prices depending on what company you use and what your needs are. Avis gave me the best price. Rentals are available at the Hess station by MK, at SOG, and the Hilton in DTD. Or pickup @ the airport if that is easier.