Would someone please let me know if they can value these 4 Nat Geo magazines?
Nov. 1985's Human SKULL-anthropological discovery,
Dec. 1988's EARTH issue,
a special Laser-imaged BALD EAGLE issue, March 1984.
December, 1988: McDonald's Holographic ad (back cover)
I have had them for 25 years. I actually have a whole collection beginning in 1970.
Any assistance is greatly appreciated!
This may assist you - though it is considerably more information than you are asking for. Nevertheless, the info you're requesting is in the midst of this document and should give you some insight:
The short answer is, nearly 7 million copies were produced per issue/month by 1970. The more of a thing usually means each thing is less valuable.
Another way of assessing your "key" issues is to search eBay for that particular issue and average the asking price for each listing, then observing the final sale price for the issue.
Hope this helps you find what you're looking for.