I would offer a slightly different perspective. A unit sitting on the lot has already had some wear due to time, weather, sun, people walking in and out, maybe it has been to a show, etc... Similarly, a built-to-spec rig will include additional paperwork and management from the dealer to get the right details in, make sure it comes off the line correctly, and more.
From both these perspectives, a built-to-spec rig might be considered a higher value (and thus more expensive) than a general floorplan rig.
All in all, like DTR, we ordered our last one (actually caught one just getting built for the dealer and had it customized), and we expcted that the price would be no higher or lower than a new model year on the lot, which was exactly our experience.