I have made and cancelled over 12 reservations in the past 2 months looking for the best price for my upcoming Vegas trip (mid-May). There is no set pattern when the prices go up or down. I have gone on line in the morning to see one price and few hours later another with another change in the evening.
I use the Costco Travel website for as it compares 4 different participating companies and shows all their discounts for every type of vehicle.
For some reason, that I haven't seen before, Budget has been coming in with the lowest rates for the past couple of days. I booked a full-size on Budget for 9 days for a total (fees & taxes included) at $143. The lowest price I have booked in the past 2 months on Alamo was for $166.
I also have a Mini-van reserved for $220 on Alamo for the same 9 day period. Budget had rates as low as $209 for the Mini-van for the same period. Today it might show $550.
Just takes time and effort.