It depends on where you're looking, identical english copies are available in other regions too.
http://www.ebay.com/itm/PC-DOS-Storm...-/360482400999
Again, maybe a buy-it-now for $100 might sell. If anyone offers you $50 you might want to take it. Whenever something is obscure and copies don't turn up often, people post their copy for high prices hoping someone is desperate enough to buy it. Other people see these listings and now think their copy is worth that much, so they post their copy for a high price too. It doesn't mean they sell, if there's no completed auctions those current high priced listings just get relisted when they expire.
It's the same with Amazon listings, tons of times I see rare stuff listed for big bucks only eventually the same thing turns up on ebay and ends at under $10. Storm Across Europe seems to be a more generic SSI strategy game from 1989 on floppy disk, I doubt many people would be actively looking for a copy. With rare items that don't come up for sale often the value is really whatever you can get for it, maybe someone would be interested as they've been looking for a copy, or maybe not. If anyone shows interest you might want to sell it to them.
Personally I have a new/sealed copy of The 11th Hour for Mac, the Mac version is supposed to be pretty rare. There's a single disc only used copy on Amazon for $43, and another copy on ebay for the same price(maybe from the same seller). I can't get $10 for it locally and I've been trying to sell it for a few years now. If anyone would have offered me $5 I would have accepted, but nobody replied for it. I tend to buy things so they won't get destroyed if I leave them behind, but I often get burned by it.