I quite like the 6"-sound, and i fits perfectly to my rather odd Gretsch-kit with 8" tom/13" floor/16" bass. But isn't 500$ too much?
If I'm not mistaken, I think the 6" double-head toms were available during the mid to late '70s.
Yes -- $500.00 is outrageously high-priced for a small tom that barely anyone every uses or even much wants. They're rare only because low sales didn't facilitate making many of them.
Tuned too high, they tended to "bap," and tuned too low, they "poinged." Don't even bother putting a mic to one, because all you'll get is a very brief attack sound - no tone.
I agree with RickP - the 8" had a great tone. Enough shell and enough volume to sing with the rest of the toms. But the 6"... Nah. They're about as useful as 6" splash cymbals.