Many things go into the equation. One, will it be a two story or one story? Will anyone be home during the day? Will it have 2 AC units?
If you will have a two story with 2 units you can just run the one unit at 75 during the day and it will cool the house and when everyone isl home at night, you can always turn on the second unit. In this case, your bill will be between $350 and $450/month. We personally have a home in excess of 4000 sq.ft. and that is what our bill runs in the hot months.
A one story with one unit - $275 to $325 - depending on the setting. If it has two units, one can always be off during the day.
I hope I have helped. Good luck! Becky