Really low-down.
 
Peter

Really cool!  
 
There was one of these at the NAMM Museum of Music Making in Carlsbad, CA.  We visited last month and there was a display of unusual saxophones.  I don't remember that one having all the engraving, but how many of these can there be?

I LOVE that tone!

Saxophone players can be really annoying( a guy who regularily plays across the street from my work comes to mind...he's way too happy-toned), but this guy sounds good...Tony

As long-time readers of this forum know, I play more saxophone than bass or guitar, so I can't avoid commenting :-).  In Peter's posted video, Randy Emerick is playing a contrabass sax.  This is an octave below a baritone, which is the largest sax commonly seen.
 
Most modern baritones can sound the C below the E string on a four-string bass, or the first fret of the B string on a five-string bass.  The contrabass is an octave below that.
 
The contrabass is, IMHO, only a novelty instrument.  It's too big to transport easily; can only be played from a rigid stand; takes too much wind for legato passages.  But it does make for a fun YouTube posting :-)!

And how much would one of them cost?? I once had a really good look at a sax players instrument years ago..very complex levers, rods, keys and pad valves so no wonder they can cost the earth.
And then there is the maintenance of them which cannot be cheap

Emerick is playing an Epplesheim contra, which costs about $45,000.  Maintenance depends, of course, on care and usage, but it's not really bad (based on my experience with everything from a sopranino to a bass).  Practice regularly and play weekly, and you're looking at $50/100 per year.

From an engineering point of view, they are an incredible piece of equipment as I suppose are clarinets, flutes, bassoons etc.