My brother is looking to sell his Folded Horn Subwoofer.
He had it professionally built by a local woodshop entirely of 18mm baltic birch plywood. - He spent quite a bit of money considering it is a challenging build.
I believe it houses dual 12" Eminence Lab12 subwoofers. It is an absolute monster from 35hz - 200Hz. With room gain, it digs a little lower.
My brother had this built knowing that it was not the last word in low-end extension, but because it was the most musical bass he had heard.
He used it with a pair of Klipsch Heresys for music and occasionally HT - and its a great sub for anyone in this group running a hi-efficiency / horn-based system
Nor he or I know a fair asking price for this, but he paid well over $1,000 for this few years back. For now, he is asking $400 (which is about $50 less than what it costs to just purchase a pair of Lab12 drivers).
See picture below from Inlow Sound: https://inlowsound.weebly.com/folded-bass-horn.html