At least someone answered the original question 
I was hoping for someone who has Steffens book at hand to chime in (if that topic is covered in there).
Until then I can only contribute my superficial knowledge:
From Veliers side, it is more or less known that there are only a few casks left. “The Last” was indeed the last release where multiple casks were combined into an outturn of 1000–5000 bottles. The remaining casks, identified as “the best” or “most special” were filled into glass (dame jeannes) and are to be released in the paradise sereies (#1 and #2 already out, at least 15 more to come, as documented in the epic velier tasting). After that, it’s more or less it from Veliers side. Maybe a couple of casks left in Lucas cellar, but overall my guess would be <20 casks.
Bristol was another major buyer, no idea how much they bought and how many are left. The number provided by @Steffmaus feels a bit on the low side, but at least it’s a ballbark. There are definetly not several thousands left.
The prices are a combination of taste and rarity. Tastewise, Veliers stuff has the edge, that why they are leading the price curve as well.