Pardon me for necromancing the crap out of this thread, but I did figure out a trick for this particular item a couple of years ago, which led me to acquiring one:
It cost me USD $181.06 from Amazon in September 2019. While the only copies currently available on Amazon are ridiculously high, at the time of purchase, they were all in the field of what I paid, and other sites that list them are possibly even lower.
The trick is that you're looking for a Japanese publication, but you're probably searching for it in English. So "Nights Into Dreams Sheet Music" will only return results for sellers who are selling to English speakers, and know how hard it is to find.
The Japanese title for this book is: 楽しいバイエル併用 セガ/ナイツサントラ (Google Translate: "Fun Beyer Combination Sega/Knights Soundtrack").
Additionally, sites are region based, so searching on Amazon.com will get you nowhere : https://www.amazon.c...id=TGWGKS6CZJRC
While Amazon.co.jp, where I purchased mine, sometimes has stock: https://www.amazon.c...˜/dp/4810826910
Granted, those prices have gone through the roof, but there are other sites.
eBay is not present in Japan; they prefer Yahoo Auctions: https://store.shoppi...3002-00083.html
That's < USD $200, with free shipping in Tokyo.
And although this listing is sold: https://jp.mercari.c...em/m82672302917
You can see the price wasn't very high. Keeping an eye on sites that specialize in Japanese game/anime publications like this will turn something up eventually.
If you are interesting in acquiring this book, start searching using the Japanese name, because it will turn up more often, and at lower prices. Additionally, the game's name is often abbreviated to simply ナイツ, or "Nights", so listings like this will also slip through the search cracks if you're too specific: https://animeartbook...?v=6cc98ba2045f
You'll probably have to use a package redirection service, like I did, but that's not too much extra, considering. Finally, Google Translate is your friend on this journey.
I hope that helps anyone who comes back here looking for information on this.