I checked a 2-day rental using NAtional, booked 12/31 - 1/2, standard size car (assuming you are driving ~1/2 way and overnighting somewhere in the VA/TN area). They are quoting a rate of $124.20 per day, using the Priceless Vacations code (6600330). Total, including taxes and 10% discount, is $266.93. They seem to have incorporated the drop-off fee into the rental rate, because the same rental with pick-up & drop-off at MCO is $ 76.24, so it's not a bad as I thought.
What airline do you have tickets with and why do you fear that they will not be flying this New Years?