David, I cannot possibly imagine. For me to give any number would be RECKLESS and if helpful at all, it would be helpful to only one party.
I can say this. Both the 11 and 15 round magazines occupy this odd collector space of an item that has precious little intrinsic value (certainly, in MY opinion.) They are strange, look horrendous hanging from a pistol, they are far too rare to justify actual use, their tangible benefit is arguably quite low, they are crazy valuable because of crazy guys like us who love interesting and rare items associated with our passion.
If I somehow stumbled across one for cheap dough, I would snatch it up and absolutely look to make fat cash on flipping it. It would have to be some crazy buyer looking for esoteric items to want to pay big money for it.
Lastly, I actually know one lunatic guy who has one of the 11-rounders and he carries it for woods carry. That dude is a nutbar, but also one of my best buddies

and I so I call him these names in hopes that he might read this post and chuckle.
This is one of those items that trades hands so rarely that whatever price you and your friend agree on, know that if you report it, you both have a fair hand in literally determining or at the least, swaying the market.
To that end, I do hope you get it and I hope that you report the price you agreed upon.