"Loved but undervalued" is probably correct. I believe the main reason is that the appearance of the new M-mount version has depressed the value of the earlier LTM lens. The newer lens is slightly smaller, and I believe the market for the Nokton is mostly for M-Leicas. The LTM lens is really too large to use on a Barnack, as it seriously blocks the viewfinder window. The market for its use on Canons, which have a VF window much farther from the lens axis, is not all that big. So the LTM Nokton is a bargain these days: good if you want to buy one, not so much if you have one for sale.
Cheers,
Dez