I dont think its right to say the mistake boils down to 1 number. The price is so far off from the pc that they are effectively completely different products for different markets and uses. The deviation is deeply rooted. It just most obviously manifests as a price
Since your comment is flagged to death, I'll leave this here for a more hands on approach for interested people:
https://interlisp.org/ is one of the systems which ran on these machines at the time.
Due to the stars aligning right in a syzygy it is now available as open-source, updated to run on modern systems. Even in a browser, without installing it.
Another one would be https://cuis.st/ which is the nearest Smalltalk-80 like thing available. Not as toyish as Squeak, not as bloated as Pharo.
Or even more 'original': https://github.com/rochus-keller/Smalltalk