Or are they? You can find quite many on Ebay. They start at hundreds of dollars (only the PCI board) and if they are complete with original box and manuals they can even pass $1000. If they get sold I do not know, but there are no cheap Ultrasounds up for grabs as far as I can see.
Are these old, second-hand soundcards really valuable? In that case: why?
I was truly sad to lose 16-bit ISA slots on motherboards because it meant I no longer could use my GUS. Held out as long as I could, disabling the audio built-in to the motherboards at the time.