I'm sure somebody has worked out an equation that will tell you what is cheapest, but I don't know what it is. I'm sure it depends on what the outside temperature is, what the inside temperature gets up to, how long the thing is un-cooled . . . and there is surely a combination of factors that argue one way or the other.
Maybe what you need is a programmable thermostat for the AC, so you could set it to back off on the cooling about the time you head out the door, then start cooling 'er down about an hour before you get back home in the evening.
I just got done replacing the thermostat in my rig, but just for the furnace. The one that came with it took the heat through a 10 degree swing between furnace on and off that made it only slightly less uncomfortable than no furnace at all, between throwing the covers off roasting hot, and shivering in a ball. I used a residential thermostat that has a set of AA batteries, and is pretty unsophisticated, but does not need a 24 volt connection. You would have to do some extra wiring and a little modification to your AC, but I bet it could be done.