I don't have an accurate comparison, but living in Houston, my AC stays on 95% of the time (even in winter, I just crank the heat up and leave the AC button alone).
Even with AC blasting, I still have a hand calculated 22.9 MPGs full highway, so AC compressor strain must be negligible. Also, my compressor has had a slight knock for the past 80k miles (i'm assuming bad bearing) so it can't be running too efficiently, and I still managed that MPG.
Results may vary, especially depending on altitude where the air is thinner. I'm 60 feet above sea level here.