Came across a guy selling one of these in a local paper for around $400. Anyone have any idea of what one of these fetches nowadays? I'm sure it's more than the asking price.

The Thunder Five was not a Colt product.  It fired .45 Colt and .410 shotshells.
ETA:
A quick search makes me think they weren't very well thought of.  I do remember one being used in the movie Three Kings.
I found a few closed listings for around $400-550.
 

My roommate owned one of these in the early 1990's.  We shot it both with 45 and .410 shells.  We spent more time showing it to people than we did ever using it.  

At anything but the closest ranges, it is fairly useless with both kinds of ammo.  45 will keyhole, .410 will pattern like crap.  If you are lucky 00 might penetrate something.

It is clunky, inaccurate, and the trigger is poor.  Build quality is actually somewhat decent and it is not made out of pot metal.  I view this as nothing more than a conversation piece.  If you want something odd to fill out your collection, go for it.  If you are looking for a shooter I would forget it unless you are trying to fulfill some very specific need (maybe a snake gun or something, I dunno).

I do not remember the story with the company that was making these.  I seem to recall they ran afoul of some state or federal laws, or the gun was banned in some states or some nonsense.   Maybe someone else in here will be able to fill in the details.

I just laughed when I started seeing ads for the Taurus Judge a few years back.