I've found when checking rates that the price can be higher if rented for exactly a week (picking up and returning at the exact same time of day). This was usually the case when checking prices through Dollar. By shorting my weekly rental by 1 hour, I often saw a 20% drop in the rate. I think many people just put in the same pickup and return time of day when checking rates so they don't know they might be able to save by making the return time a little different.
I went to Dollar's website, picked a day at the end of this month, and found that the rate dropped by 10% when I entered times for a 23 hour rental verses a 24 hour rental.
When using Costco, I've found the lowest rates with Alamo. I've paid as little as $35/week plus taxes for a standard size car with them on Maui.