What is the cost of a fat transfer to the forehead and why does it vary so much between doctors?

I'm looking to have a fat transfer to the forehead to re-volumise so lost fat/collagen. I've been quoted prices between $1000 to $5000. Why is there such a range in prices? Is it to do with expertise or age or something else? I'm currently 26 and looking for a boost to the lost collagen. Thanks for your time.