I am not going to be very helpful here, because I am the maths-challenged one in our house so don't handle the bills.

I can tell you (because we only got AC last summer), that apparently it's best to err on the side of a big air conditioner, because a large air con won't have to work as hard as a smaller one. Like, to maintain 24deg inside a smaller air con would have to work harder, and therefore use more energy.

We have a 3 bedroom, one study, open plan sort of house and the one unit cools the whole house, the air flows from our rumpus (we direct the air that way) through the kitchen/dining, and from there it makes its way to the front lounge and cools our bedroom a bit. The hallway to the boys room comes off the kitchen/dining, and usually we just use a fan in their doorway to pull the cooler air in a bit. They sleep happily without blankets and clothes on the hot nights though, and enough cold air does get down to make things comfy for them.

I think with efficiency, it's apparently better to turn the a/c on before the house gets hot (probably a no brainer for many) - and I think every degree under 21(?) gets more and more exy. But I think (this is just from listening not very well to DH), that it's cheaper to run the air con most of the day at 25, than to turn it on at 20 when you start to feel hot. I think.

I don't think it has been outrageously expensive. Having said that, he is the one who feels the need for the air con more, so he is more likely to stay quiet about the cost.

There are heaps of things you can do to ease the air con's load as well, like making sure all windows are shut, closing curtains when you can, planting trees/shading north and west facing windows if you don't have good eaves, good insulation, turning it on early, closing doors to rooms you don't need cooled.

Anyway. Probably all of that is common sense or not useful to you, but DH went through a phase of figuring out what would be most economical and talked about it for ages, so I may as well pass on the little I absorbed.