Peravia Barograph Question

Friends:

We recently received a donation of an excellent condition Peravia barograph
(with case and extra paper). I promised the donor a letter stating the
value of the donation. There was a time when these were very valuable -
but I do not know if they still hold that value in these days of GPS and
secure data recorders.

I would appreciate opinions of value or recent sale information from
experienced people on the list. I personally own a different Peravia but
rarely use it now.

Thanks

Roy Bourgeois
President, Greater Boston Soaring Club