Great Notion: Double Stack

Can't help you with your question regarding trade value, but thought I could provide you with some information about the beer itself.

GN doesn't bottle any of their beers, but they do periodic crowler releases, which is what you would be trading for (a crowler of Double Stack). As I recall, crowler pricing for the DS is in the $15 to $16 range.

GN brews DS all the time, and it's usually available on tap at the brewery. Unfortunately, at least for now, crowler releases are pretty limited, with GN typically releasing around 200 crowlers (2 per customer limits) several times a week. In the past, they used to post upcoming releases on their facebook page, so you could plan in advance when crowlers of DS might become available. However, I haven't seen any recent postings about upcoming crowler releases, and their website doesn't provide any info either.

You could always give them a call I suppose (or email them or message them on FB), and ask them when they're next going to do a DS crowler release.

Good luck.