Are these rare objects?
Hi Deltab-- yes, they are rare, but they're not something widely collected, so competition is low-- resulting in a market price of about $50. I have two in typical light aqua; they also come in clear and green (both rarer, as are half-bricks).
So, if it's your kind of thing: a great find at any price, and IMO a steal at $50. Congratulations, you have a nice early piece of glass architectural history. To most people, it's some old junk and $50 is way too much.
They are sometimes mistaken for glass floats. In that market, going price is more like $200-$500 depending on color, based on rarity plus a larger (but mistaken) market.
There is at least one building in the Netherlands (neighbors to Belgium where the blocks were made) which still has them in use!