My left side of my butt is sunk in really bad and the right side is perfect. . What procedure is best for me?

Hello dear!!
Thanks for the question and provided information as well.
Looks like you could benefit from a fat transfer procedure, but unfortunately, without pictures, it is very difficult to give you a proper answer. 
The average cost varies a lot depending on the clinic, the country and the plastic surgeon. l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

If one area is flat and sunken in, then perhaps fat grafting may help to enhance the buttock to alleviate the depression.