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

In order to determine the best procedure for you, photos would be very helpful but an in person consultation with physical exam best. Depending upon the severity and relativity compared to the "perfect" right side, fat grafting or an implant will likely be the best options. If the left is significantly smaller, then a buttock implant is the only reliable and predictable long term option (at least in my experience). Glad to help. 

Without photos it is difficult to say for certain, but from what you describe, liposuction and fat transfer would likely be your best bet.  Good luck!

Without pictures, one cannot say for certain what would be the best option to correct the problem. Grafting fat harvested from somewhere else on your body may be a good option, but if the defect is too great , an implant may be required

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.

Thank you for this interesting question. I think the right procedure for you would depend on how dramatic is the left buttock concavity and the location of this deformity. I would also like to know if this was congenital or traumatic. You have three options:  augment the left side with fat grafting, augment with an implant, or use both techniques if the difference between the two buttocks is very large.