The trite and unhelpful answer would be, whatever anyone is willing to pay. The more honest response is that I haven’t a clue. It seems a difficult time to sell this kind of item. Having had a couple of conversations lately with people interested in vintage and historic arms both told me that they just couldn’t find buyers for rifles and accessories that a few years ago would have sold easily. It does appear that the market has declined in a similar way to that of lot of the small antiques and collectibles. A good example is Goss Crested China, the link gives a good account of what has happened.
Looking for similar items on Ebay usually turns up a wide range of prices, take a look at these for The Bell Medal; 56,000 of them struck! A collector is unlikely to pay more than £5.00 inc postage, maybe £10.00 if there’s something special about it. That doesn’t stop the speculative asking prices.
Coming back to your sight sets, they’re not really in demand and they do appear quite frequently. Try a set at £50.00 and if they don’t sell try a couple of months later at £25.00. If they go put the next set on at £35.00. You could however ask £40.00 for the pair, I would have thought they might go for that.
It’s a bit of guessing game, hope this gives some food for thought.
Rutty