Turnbull has spent his entire life developing his procedures and processes for restoring firearms. He has a world renowned reputation for the quality of his work. There is literally no one who approaches his abilities or results.
This places him in a position of market control. Yes, he has a lot of work so in order to control the workload, he raises his prices knowing that he will get only the cream of the projects needing his attention.
His concern is not for what profit you can achieve but rather that he is compensated for his knowledge, time, efforts and abilities. If your potential selling price is not up to your expectations of being enough profit, you had the wrong person complete the project. Maybe there is another way to get the results you want without overpaying.
Just like picking a gunsmith for your dream rifle or handgun, you have to balance quality and price against delivery time. There are lots of top quality gunsmith here in the U.S. but seemingly everyone wants to use the top 2 dozen for name only. Well, buckle up because they only have so much time and if the workload gets too big, the time frame and the prices go up.
Maybe you should allow someone else to handle your refinishing project?
Best of luck with your project!