The effect is noticeable but not huge. There is no universal estimate. Different cars different carriers, enclosed or open all make differences. Then there is the question of being able to remove the carrie when not in use, your personal driving habits. specific numbers will only apply to the specific car the specific speed of the car, load in/on the carrier etc.
Back in the 70’s when I had a carrier on my VW Bug, the empty carrier did produce some wind noise, but no noticeable mileage change, I seem to remember about 1 mpg on my car that averaged just under 30 mpg.