Can't answer the question about cost but I can give you an idea of what we spend on running our AC during the summer. It's not much. Our house is 3200 sq ft and temps in the summer here are frequently in the upper 90s, and maybe a 15 days or so in the 100s...up to 115. I run the AC constantly in the summer and keep the thermostat set at 73 or 74. I frequently run a room AC in the bedroom at night when my wife is working. Our house is entirely electric. The AC adds $40 to our bill during July and Aug, $20 during June and Sep. Pretty cheap comfort if you ask me.
Having said that, our electricity cost here is only .08 per KwH. Cost in Bailey was about .18 KwH. So it's much cheaper for us (thanks to all the hydro power in the area).