IMO Pricing is the only factor to determine whether a product is an upgrade to another, BY THE MANUFACTURER'S STANDARD. Senn's "ability" to produce HD820 should be about the same as its "ability" to produce HD800S, thus the supply is unchanged. Pricing the HD820 higher reflects the fact that Senn predicts a higher demand for HD820 than for HD800S.
...or a lower demand so the need to compensate by pricing it higher? Or possibly that plus somewhat higher manufacturing costs due to that concave gorilla glass?