duanedr 

There have been several blind 'semi-scientific' experiments that show tubing has a very small and non-intuitive effect on the ride of the bike.

Assuming we aren't talking about XcR or 953 types of tubes, the tubing is a small % of the costs that go into a frameset. The difference between the top and the bottom of the price range is irrelevant (5% of frame cost?) to the price of a frameset. The paint can be 20-50% of the cost so, 5% for tubing isn't a concern.

You are paying for their expertise in designing a frame that matches your riding style, weight, fit and the kinds of riding you're doing as well as the fabrication skills and warrantee support should you end up there. Ask questions to understand their design choices, not 2nd guess them. If you are 2nd guessing them, you haven't found the right builder for you.