I'm not so sure, I waited ages for one to turn up on either Yahoo.jp or Ebay and none ever did, I've seen all the other rare games but not a single Sylphia.
Then I thought I'd wait untill I next go to Japan and pick one up then but then I heard the price in Japan had started rocketing (also confirmed by Tatsujin).
I read that Sylphia had a limited print run and I can only guess that now all the copies have ended up in the hands of collectors who aren't selling so the demand has outstripped supply.
I just tried to find a little article I'd read about how Sylphia had become rare and expensive in Japan over the last year or so but can't find it again now.
There are 3 up on eBay right now. I guess it's entirely possible that for over 10 years I was one of the only people with any interest in the game. But even games that people don't like but are rare were selling for ridiculous collector prices before that. The market is always fluctuating and recently Sylphia has jumped up in price with collectors, likely due to an increase in the number of PCE collectors, especially particular ones. Just as non-rare U.S. Turbo stuff goes for an arm and a leg because of particular collectors.
Since it's now priced out of the range that any real PCE player would pay, it's simply a commodity. Although collectors who collect just for the sake of collecting are the problem, these trends always die down after a while and there are always people quitting or getting tired of it. Especially once the number of real PCE collectors from the generation who grew up with the games starts to wane, there'll likely be a dip in overall prices.
Thanks to modern technology, it becomes easier everyday to buy, sell and learn about games, as well as so many former exclusives to get re-released in various forms. So the games themselves are only becoming more available, even if there are only so many copies published in the PCE format. De-mystifying many "rare" games often helps depreciate their value.
I've seen radical shifts for almost every PCE game over the years and I wouldn't be surprised by anything in the future. But for years I saw few available copies of games like Sapphire, Renny Blaster, Fausette Amour, Kaze Kiri, etc... but there were always copies of Sylphia everywhere and there still are. It's not like a bunch got destroyed or anything. The only thing that's changed is what people charge for them, just like U.S. Turbo stuff.