With the right codes and discounts, probably around $16 to $20 per day average for total costs of 5-day to 7-day rentals. I feel good when the total price (rental rate plus all taxes and fees) averages out below $20 per day.
Book the best rate you can find but keep checking regularly and follow the thread recommended above. Most of the time the rates will drop a month or even a couple of weeks before the trip but it really varies.