As far as the largest known moldavite, there are different weights mentioned on various websites. However, this compilation at least lists its sources:
According to this work, the heaviest known moldavite was recovered in Bohemia and weighs 500g! The source for this info is the 2001 book “Tektites” by Ken McNamara and Alex Bevan. This website also lists two other moldavites found in the Czech Republic, which weigh 265.5g and 258.5 g.
This website, https://www.tektites.info, seems to have a great deal of information on tektites, including moldavites. Their contact page lists some Facebook groups, where perhaps you might find people with specific information about the most expensive moldavite piece sold, estimated time to depletion, and the estimated gem and crystal market/sales in the UK 2020/2021.