On the Costco site today 3/26-3/30 0800 pick up 0800 drop off intermediate car ~215 all in.
3/26-3/30 0800 pick up 1700 drop off intermediate car ~156 all in.
So pricing is about the same the only difference being not having to sign up for the Alamo Insiders Club by going through Costco. The downside being you will have to input your information at the kiosk or with an agent. I would sign up, book, and check back often to see if pricing drops. If it does, cancel existing reservations, wait about 5 minutes then rebook (as long as you did not prepay and not within the window in the T&C's).
Or you could priceline it...