I know nothing about the market, but would guess that db shotguns tend to be a bit spendy for some reason[s].
We have plenty of the $1000-$4000 over/under and side by sides, but every pump and semi-auto is gone.
They usually have nice engraving and fancy wood for the stocks, so they cost considerably more than utility shotguns.
DB shotguns require special fitting to make the two barrels thus their shot to converge at some predetermined range. The process is called "registration" and that's what adds so much cost to DB's. Sometimes they are "fitted" to an individual shooter, because shotguns don't normally have rear sights (the shooters eye functions as the rear sight and the gun is pointed rather than aimed) which of course adds expense.