How much does it cost on average for an adult to get a name change?

I want to get my name changed due to the fact that I was named after a man who has never really been in my life. My name is Mark Fitzgerald Stephens JR, I want to change it to Mark David Lawrence. David is the middle name my mother wanted for me but didn't get a chance to put down since my father did it himself. They originally didn't agree on Mark Fitzgerald or the Jr part at all. In addition, my mom remarried whenever I was like 10 and my stepfather has been like a dad to me. His last name is Lawrence. I was wanting to change my name to Mark David Lawrence. How much will this cost? How lenghtly is the process?