There was a recent ebay auction for one in very poor condition. It's very uncommon to find any 19th century supplements in the actual publications after all of these years. And it certainly helps that yours is in exceptional condition. There are so many supplements out there where the origins are unknown because they're removed from the original publication.
I initially was thinking that price range would be $150-$200 but I'm rethinking that based on an old Hunt auction below. Great pickup regardless of value!
Hunt Auctions
1896 Boston BBC Supplement