I tried it for 3 different trips. For each trip I first got my corp rate, the Travelersadvantage rate and lastly the Costco rate. Took the lowest one, and bid 30% less and was rejected in 1 hr. Ditto for 20% less and 10% less. Then called Budget and they said they had plenty of cars avail at each location.
My conclusion is it is a waste of time to attempt to rent a car by this bid method.