Jerome Fine replies:
 It may also depend on how many PRO380 systems are still
 around as opposed to PDP-11/73 systems.  I often found
 PDP-11/73 systems available for tossing up to about 5 years
 ago. 
Now there is an interesting, and very valid point.  Personally the only
DEC PRO system I've found was the system console for a VAX.  Now that you
mention it, I don't think I recall seeing any available in recent years.
At the same time this could partially be a geographical situation (even with
eBay, as most systems probably don't end up there, insted ending up at a
scrapper).  Recently someone commented that a PDT-11/150 was pretty common,
however, he was in the heart of the old DEC country.  It's been my
experience that PDT-11/150's are anything but common with PDP-8's being more
common!
I maintain the way to go is to simply start collecting the parts needed to
build a Q-Bus system.  Remember a MicroVAX II can be used as the chassis
(that's what the BA123 I use originally was).
                Zane