First, I have a professionally designed and built layout. I considered Dunham Studios but for certain reasons, mostly it is S scale rather than O, I chose a competitor. Dunham appears to do great work, as does TW Trainworks.
The issue Lou may have is these firms build the layout in their shop, test it, then disassemble it, ship it and reassemble it in the owners layout room. So the owner will not be building the structure. If the plan is to have the layout built in the layout room on owner supplied benchwork, there may be some people who do that but not the major custom layout builders. As far as the builders location, if the layout is shipped, the cost difference for 100 miles vs 1,000 miles is rounding error in the layout cost.
As far as a likely cost for a professionally built layout, the numbers mentioned above are achievable if the scope is tightly controlled but it is easy to specify a layout that costs a lot more per square foot.
