How often and how does it cost to flush the transmission fluid

I would change it every 30,000 miles If your transmission has high mileage and the trans oil was never changed I'd change the filter a well but the trans pan and filter are on the side of the transmission and take nearly 2 hours to change. Straight forward drain and refill will take 3 to 4 quarts of Mercron fluid, $4 a quart, $2.50 hazard waste disposal, .8 hour labor.