Does anyone have an idea of
the value of this?
I did some Google-Fu and found a posting on Reddit from 3 years ago where the guy said he paid 150 bucks for a factory 4-pack.
If I understand correctly, Checkmate was the last supplier of M-14 mags during the GWOT. They list mags at 54.99
M1A / M14 Magazines - High-Quality USGI Specs | Check-Mate Magazines
I would expect that 4 original mags in the wrapper might have some collector value - if you can find someone who collects such accessories.
If you are looking to sell them, I certainly would not sell them for anything less than 100 bucks - and that would be fire sale prices. $150 to $200 seems reasonable.
Chinese made mags for $18 here - just to compare.
M14 Magazine (Taiwan T57) - 20 Round 7.62x51mm NATO | M1A Compatible | Keep Shooting
Hope that helps.