Fair Value for a '28 HighBoy Roadsster

[russcc]

A long time close friend has made an offer to buy a an original '28 roadster on an original '32 frame built as a highboy and set up as flathead powered in the Vern Tardel format, following the Ford Hot Rod book by Vern & Mike Bishop.
The roadster is setup with '40 running gear, original Mor-drop axle, F1 steering, tube shocks. The hood, side panels, and other body components were fabricated of aluminum by a well known New England craftsman and vintage open wheel race care restorer. There is not much data to base a fair value on. The roadster is being sold less engine. We would appreciate you sharing your experience. Please see photos. Please PM with questions. Thank you.