body and paint is not easy, therefore it's not cheap. if it were easy, you would do it yourself. $425 to paint a tailgate is about right, it is more work than you think. the thing about those guys that come out and do spot repairs for dealerships is that they either will do it for cheap and it will look cheap, or they will do it right (as right as they possibly can do it while spraying paint in a parking lot) and it will be expensive. they do work for dealerships and whip out a bunch of cars in a day but the quality is typically low. they do just about a good enough job that the potential customer looking at the car won't notice it, but if you knew it was there and knew it was repaired you would notice the difference.
if you want it done right, pay the bodyshop to do it right. if you want it cheap, a touch up pen is your friend.