With my '96 Ford Contour, I see about a 2 mpg loss with the AC on. I see a gain of 2 mpg with criuse control. I would say modern AC compressors use about 2-3 HP. I was told the old York style compressors consume 10-15 hp to run. It has been a while since I've run my '75 Bricklin, but I do remember a big power loss running AC. I can note some amount of power loss with AC on, on newer cars, but not that amount.
Also, fuel can have an impact. I have notice a +2 mpg with some gas. Also, if it has 10% alcohol, I usually lose 2 mpg.
Tom