In a country were 30,000 plus sales are extremely good for an anime. Discs that cost 7,800 yen retail. It isn't like buying a $20 or 30 dollar film at Target. These things are expensive, and they have always been expensive in Japan. It is a specific market of people that buy discs over there when renting is popular and there are legal streaming sites.
And yet Yamato still sells very well compared to other anime even when it has less extras than some of the others. Because it is Yamato.