The dongle was a niche thing and Apple seems to have discontinued it, and it appears that it was never cloned by Chinese eBay sellers, so if you want to use Firewire gear with a modern Mac you're stuffed.
Yeah, I had a hunch that audio was probably the reason behind it. Even I have a Behringer thingamajig somewhere but, as TheShortTimer has pointed out, there were other, possibly better, alternatives out there which are not fetching as huge a price as this dongle and I couldn't work out why this adapter, in particular, was hitting record highs.
I mean, I picked up a Thunderbolt display for peanuts recently to use with my Trashcan and it has a built in TB hub including a FW800 port, as one example.