The Selkirk glassworks has shut its doors, so there won't be any more Selkirk paperweights being made. However, the eBay effect has depressed the market for many types of paperweights (most contemporary sulphides are hopelessly inexpensive) and for some manufactuers of glass, Selkirk being one of them.
Putting a future value on your weight is impossible at this stage of the game. Based on the true retail cost of its paperweights - when Selkirk was making its weights, accept that you got a very good bargain. But also acknowledge that the price is not going to go up any time soon. The value is what you paid for it.
Selkirk's weights are fanciful and well-made with wonderful execution of design and colorful styling. Unfortunately, the world of eBay works against higher prices for mid-range paperweights.