When there are few or no sales for comparison, it's hard to pin a price on something. You can do as realtors do, and look for "comps," i.e., values for similar items, but that's really guesswork.
The ETF page mentions that these tubes weren't the greatest (convergence problems). I wonder if that means that few were made? Before plunking down big bucks, I would try like heck to test this tube, and also do some investigation into what other tubes, if any, might be used as a sub. I'm not one of those people who enjoys owning non-working doorstops. If there's no way on Earth to make a set work, I don't want it at any price.
Just my $0.02.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html