Had a great example of this last weekend.
Booked a 3 day rental at SNA using the AAA CDP and a $10/day AAA coupon.
On Wednesday when I booked - $56 total
On Thursday night when I re-checked - $41
On Friday - $24
When I actually returned the car they botched the coupon and gave me $40 off instead of $30, so the rental ended up being $11, but even without that the prices still halved in about 2 days.