Fences run from 4 feet or so in front of the bullpen in Fenway to 37′ 3″ for the green monster in left.
The brickwork at Wrigley is 11 1/2 feet while the Coliseum has 15 foot walls in the power alleys but eight feet down the lines.
Minute Maid Park’s walls are all different heights Left field - 19 feet, Left-center - 25 feet, Center field - 10 feet, Right-center - 10 feet, Right field - 7 feet.
Turner Field was eight feet, four inches all the way around but the wall at Sun Trust Park wall to be only six feet high at the left-field foul pole; eight feet, eight inches high in left-center and center field; and 16 feet high in right field, including at the right-field foul pole.
Googling each ballpark will give you the wall height. Calculating the average will be your math homework.