If complete human skulls cost $45 a piece (or maybe $90 a piece, as is expected in the 21st century, the story isn't very well written) and a partial costs $600, it seems that there's a pretty good market out there in converting full human skulls to partial human skulls.
The "partial human skulls" have undoubtedly been ornamented with semiprecious stones, carvings, etc. Perhaps repurposed into drinking goblets.
Regards,