My old house leaked like sieve (until we replaced the windows) and had a 10 year old A/C unit and had 40 year old insulation, and at 1500 sqft we were bleeding money to cool in it the summer but it still topped out at around $250.
My current house of 3080sqft was built in 2000 has a/c units that were installed in 2000 and we project it to be about $350/month in the summer- we were at $80 in the winter. If I replaced the a/c units, which I will do next year, I'm betting my money utility bill drops to under $300 in the summer. I have the equalization plan with SRP and they currently have me paying $173/month but I suspect after the summer audit that will go up to around $250.