Hi,
Vin asked:

I have come across a very nice 799 with all the fixins', what is a realistic
price to pay for this unit?

You couldn't go too far wrong for $100, though of course $50 would
be better.

I think it tests for mutual conductance, would this unit provide useful
accurate measurements?

It does, but with the proviso that it tests all tubes as triodes,
so the "Gm" figure for pentodes is only useful in comparison to what a
good tube would read on this model. However it's a perfectly valid
test method.

I don't believe that updated tube data was issued for this model,
or if it was, it's rarely seen. This was Hickok's answer to B&K,
intended primarily for TV service shops, but I don't think they sold
many.

Cheers, Alan