Hi LG,
To get a good idea of the current market value, type in what you are looking for on eBay, then when the item comes up, scroll down on the left side towards the end and click on Sold Items.
You will now see in green the dollar amounts this item sold for in the last two months. Bear in mind that a single tube fetches less than a pair since most people need two tubes.
Here is the GEC 6AS7 activity:
https://www.ebay.com/sch/i.html?_from=R40&_nkw=GEC+6AS7&_sacat=0&rt=nc&LH_Sold=1&LH_Complete=1
You will see that the range is between $120 -$335 for a single (most sold in sets of two) depending on condition etc. (Shipping calculated into these prices)
Re the GEC 6080 there were some bargains to be found recently, the range being $15-$108 for a single, but sold in pairs or quads.
https://www.ebay.com/sch/i.html?_fr...&_odkw=GEC+6AS7&LH_Complete=1&rt=nc&LH_Sold=1
You may also be able to find better prices buy looking for the same tubes under other designations or rebranded labels - there are many. I have seen the GEC A1834(6AS7) selling for $50 (Ultron brand, but watch out for Russian fakes - you have to become familiar with the internal construction to know if it is a Svetlana or GEC) and unbranded GEC CV2984(6080) for $13 a piece.
PP - patience pays.......