"Why" are Hydrogen Alpha filters so expensive?

I'm just curious what in the process of making a hydrogen alpha filter -
makes them so expensive? Why would a plain solar glass filter be about
$100 - but the same size piece of glass for hydrogen alpha is a few thousand
bucks?

Or is this really more of a supply and demand type of thing? Is it just
that so few companies make these that they can get away with charging a
fortune for them? I could see charging more for them but by a factor of
20 - 40?

Just curious. And jealous of those who can afford one of these. :-)

Glenn