Without the the Richard Black pedigree and without the extra shaft, a simple conversion like this is probably in the $600 to $900 range. with the Richard Black name and extra shaft, I would estimate true market value today at $1200 to $1800.
These are fairly wide ranges, but I think also a way to start at a reasonable asking price and see where the offers come in- if the conversion were much more elaborate in terms of inlays and joints ( materials- pearl, ivory, silver, etc.) ; then I think the range would go into the $1500 to $2200 area.
Just my take- I tend to be conservative on my market estimates, as I like to value based upon a quick sale, not a sit and wait months for a pipe dream.