Those prices are indeed the usual, but sometimes there's exceptions. You go months or years at a time without seeing individual hakka* CDs available for auction, then all the sudden two different sellers show up with the same CD in the same week. It often happens with other things I'm looking for. The second rare-auction-of-the-week will sometimes close for
practically-stealing prices compared to the first.
I also have a Satsuki solo-career CD, called "Asian Folks". It was only 2500yen.
By the way, how did you find that closed auction? Were you watching it before it closed, or do you have a special way of finding closed auctions you've never seen before, after the fact? If you check the bid history, chaos_japan_smj was a competing bidder - that's Shopping Mall Japan bidding in their name on my behalf. Obviously I didn't win that first one, not only because I wasn't going to pay more than 6000yen, but the auction auto-extended from the other bidder changing their mind about how much they were willing to spend. That's what's so nasty about Yahoo! Japan Auctions.
As for the 15000yen auction, I believe that seller used to have the same collection with one or two of the missing Heart Bazaar singles, and they were charging 20000yen. That was around for at least a month, until they gave up. The 15000yen auction has since then been relisted quite a few times too. Meanwhile if you're new to collecting Heart Bazaar,
this is a bargain. Too bad it doesn't include any of the albums, but it's much easier to find the two albums separately, than all of the singles. "Ai" alone is worth 2500-3500yen, according to previous auctions where I've seen actual bids. Digitalis is also more rare than most aspiring collectors know - you'll see one seller with a sample copy that they keep relisting, but you only see real copies on their own 2-3 times per year, and most of the bidders you'll compete against at the time don't seem too serious about it. 5600yen is almost worth it just for Digitalis and Ai together, while the other two singles are practically free. In some seasons Collector is impossible to find too. I would say the auction's true value is 7500yen. The other auction isn't as good of a deal, as its true value is 16000yen, but if you really want that hakka CD, then you might not have much of a choice. You'll have a much easier time finding Kitakaze to Taiyo and Collector on their own (to complete your collection), than the hakka CD.
*: I could be wrong on this romanization