Alas, you’re not alone in finding Hadeland unresponsive; they no longer have a full-time curator, and time- and work-consuming requests have low priority. Unfortunately, Hadeland is also where the answers to your questions lie. I suggest you try again, this time as high up in the Hadeland food chain as possible.
There is not a bustling market for Lars Fletre glass, or indeed for any of the free-lance designers of his time. He and others are sadly forgotten, not least because they «only» designed the decor to vase and bowl shapes designed by Hadeland’s regular designers. (He is listed as designer of one decanter in 1947, named «Greve» - meaning «Count».)
Catalogues of the time is of no help either, as they show the different models and designs, but do not list the designers’ names; that only came with the art glass of the 50s and 60s. The value of Fletre’s pieces is thus next to impossible to set. Having said that: from $50 to $200, for the ordinary to the absolutely extraordinary stuff.
It turned out I had three Fletre pieces in storage, posted below.