First, welcome to the BB!
What you are describing is indeed rare, and “off the charts”, so to speak… This car is very hard to replace, at any price, given the all original factor, and the fact that there are just not many of them to begin with. Let me explain. 1971 was the only year that Alfa produced “Kamm tail” (“square tail”) Spiders for the United States, with the 1750 engine. The 1750 cc engine, with the more aggressive camshaft set-up yielded a car that is “zippier” than Spiders from later years. Furthermore, no Spiders were imported in 1970, and very few were even produced for the rest of the world, as Alfa was transitioning from the round tail to the square tail. Then in 1972, Alfa went to the 2 liter configuration, with a less aggressive camshaft set-up. (The United States has always been the primary Spider market.)
For price comparisons, the ideal would be other 1971s and as a second choice the prices from other pre-smog era “Kamm tail” Spiders, which are 1972 to 1974 models. The most recent sale (Nov or Dec 2007) of a well kept 1971 Spider was by The Auto Gallery in Leesburg, Virginia, with the seller reporting a transaction for $23,500. The car was in excellent shape, however it was not original. Most notably, the SPICA mechanical fuel injection had been replaced by carburetors, so from a concours perspective the loss of a significant historical feature does negatively affect its value. Other smaller items, like the wheels and radio had also been replaced with more modern equipment.
As far as the rest of the series 2a Spiders (up to 1974), which have 2 liter engines, good well kept examples also go in the mid twenties, but there are not very many well kept examples that come to market. Exceptional cars will go up into the higher $20k range, and rough cars are hard to sell even at $5k. A word of caution here, excellent original Alfa Spiders do not typically come to market, but change hands in private transactions as usually there are friends of the owner already lined up to buy his car should he decide to sell. When this is the case, offers in the $30k range are not at all unusual.
Last, I have no knowledge or interest in this situation as I have no idea of this particular case. My comments are based on many years of being an Alfa Spider owner and following Spider transactions. Furthermore, I do have experience judging concours Spiders, and two of the Spiders in my signature hold current documentation for being 94.7 and 97.8 point cars.
Best regards,