Hi Bill,
you can search past prices on ebay.com by entering your search term at the top on the start page and then scroll down a bit on the result page and tick the "Sold listings" box under "Show only" on the left side.
If you then click on some item in the list, it will tell you bidding has ended and suggest whatever it thinks you might want, but there is a link named "See original listing" to the right of the head line. This brings you to the listing page as it was shown at the time of sale, usually including pictures if it's not too old.
I did it for an Mk32 and the highest prices were around $150 for a pair in ok used state including case, straps and filter, but not cleaned & serviced.
http://www.ebay.com/itm/1943-US-NAV...D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
The lowest price was $30 for a very beat up pair with obvious defects:
http://www.ebay.com/itm/US-Navy-194...D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557
Joachim