I could be wrong about this, but I think I remember a Japanese site that said that the PC98 version was released first, and it had the biggest sales push behind it. The DOS version was sort of an extra that came later. That's why it's so rare.
Right now, there are none of these on Yahoo Auctions, but I can remember seeing one of these sometime last year. It went for well over 10,000 yen, IIRC.