WOW!!!! Some of you would save some serious money if you would get new AC Units.
Ours is set at 69 all the time and our bill usually runs around $125.
However we got a new system last year....before new system we had it set at 73 and average summer bill ran $250.
The system cost around $2400 - so it has already paid for half it's cost - by next summer it will have paid for itself.
Ours is a new unit. I think part of the problem is that we have a very open floor plan. We don't have any enclosed hallways, and a 2-story foyer with much of the top floor open to the foyer below. I love the openness of it, but I suppose it's not very energy-efficient. Although when we installed the new unit year before last, he did make some changes to the venting system to help with the airflow.
Before we got the new unit, the upstairs wouldn't cool much below 80 at night. I slept downstairs all summer long. I cannot sleep when it's hot.