My last car didn’t have AC or sunroof, so it was a choice between fan and/or windows only. Given the amount of noise produced, the windows tended to be closed and the fan kicked up a notch (though it was already being backed up by air ram-scooping through the vents) around 50-60mph. Not sure it saps a huge amount of economy even so – it wasn’t a hugely powerful beast (topped out around 90mph) and times I actually had the windows down and my foot to the floor it didn’t seem to be appreciably slower, which is what I’d expect if the drag was increasing worth a damn (a 1hp effective difference would still have been just about noticable).

Certainly at city speeds the window venting is basically free, and better than the fan without high-speed airflow backup. I don’t think my current car’s AC draws more power than having the windows down at it’s higher top speed would, but it still makes a noticable dent in the idle rpm when turned on, until the idle control kicks the throttle open wider and it picks up speed again (and the car’s good for anything between 20 and 30mph in 5th with no throttle input depending on it’s chosen idle speed – so it’s making a not inconsiderable amount of power in this configuration)

the best thing you can do is experiment with your own car, particularly if it has a trip computer with MPG readout