It will vary greatly depending on the time of year that you are heading to WDW.
Peak travel times like Christmas and Easter will be much more expensive than say Sep/Oct. If you are traveling during off peak times, you shold be able to find something for around $30 per day.
Travelocity has a great search engine for car rentals. Give it a try to give you a ballpark idea for prices on your travel dates.
Lisa