I just rebooked with Alamo and have a midsize for May 10th-20th 10am-3pm (eleven days) for $210.77 all taxes/fees included, using the Alamo Insider, Costco code 7014627, and visa coupon AD2338FDF. That's $19.16 per day, compared to the $22.75 per day you are paying at Dollar.
Edited: Well, just went and put your dates in, using the exact same codes I used and the price jumped to over $400.00. I thought there is no way it would jump that high in just a few hours, so went back and put my dates in to check. I once again got $210.00 for my dates. That doesn't make sense. The only thing I changed was the dates (backing the pickup back one day and moving the drop off up two days). We'll be down there the same week but I am flying in on Friday morning and out on Monday evening, so will be there three more days than you, yet my rate for Alamo is almost half what yours would be? Maybe, because of the specific days I'm picking up and dropping off the car? Anyway, I just checked going directly through the Costco Travel site and found a midsize through Enterprise for your dates for $187.35. That's using no other codes, just booking through the Costco Travel site.