I cannot speak to the cost difference in your location, but in my practice these procedures are used to address similar but different areas of concern are not interchangeable. I would recommend having an in person consultation with someone familiar and experienced with both procedures.
The first thing I would recommend is speaking to a a board certified plastic surgeon in person about your goals and concerns. Weight loss after bariatric surgery affects every persons body differently. Without knowing more about your history I will say that the "donut" lift with fat grafting will provide an extremely modest improvement.