[Tfug] OT: "geek history" - what's an HP-21 calculator (1975) worth?

[Jim March]

I have one of these in what appears to be cherry condition:

http://www.hpmuseum.org/hp21.htm

I've done some reading and it appears I can test it with a couple of
standard AA batteries.  The original ni-cad battery pack is present
and still has the original batteries sealed up in it - by some miracle
they never leaked!  It also has the original case in mint condition.
It doesn't have manuals or the AC adapter.

This is the old-school red LED style.  I assume it's not as valuable
as an original HP-35 but then again, it appears the 21 wasn't made in
as much volume, and that HP shifted over to LCD tech not long after?