High 30s doesn't surprise if the rebuild was done properly, and the interior design is tasteful. I paid nearly half that for a "well used" one with significant needs.
You will have to appraise all the work done and dollars spent making it a new trailer. Hopefully the seller has photos and receipts of the work done. I would expect to see new axles, brakes, bearings, tires, belly pan, insulation, no rust or cracks in the frame, no rear end separation, new black and gray tanks of larger capacity, nearly perfect body without damage, verified no rain water leaks, new appliances, new plumbing, new 12v
electrical including battery, new soft goods (mattress, cushions, upholstery, curtains, etc.) awnings perfect or new, doors and windows all work, rubber seals new, AC new, front window rock guard and everything else?
High 30s buys a new box trailer of the same length. A vintage Airstream is more fun. I believe they will hold their value and last longer than a box trailer.