Not only they are very rare, but their owners are aware of their increasing value and don't sell them. It's such a small niche that we know individually , through the informations circulating between Alpina Clubs in Germany , Switzerland, and now France, who has one. I personnally sent out Emails to all owners I knew, asking to buy their B7S, without success.
I would pay +50,000 € for a B7S in excellent condition.
FYI,
-a green B7 turbo E24 serie III (thus no B7S), was sold for 47,000 € in France earlier this year;
- a B7S E12 is for sale at 39,990 € in germany (see
mobile.de - Deutschlands größter Fahrzeugmarkt. Suchen, kaufen oder verkaufen Sie Neu- und Gebrauchtwagen)
- a B7S E24 was for sale in Japan two years ago, around 30,000 € + transport and taxes. I sent a fax to confirm my order, but the next say I got a notice that the car was not for sale anymore...
IF such a nice car comes on the market, I doubt it will stay for sale more than a day or two. There are dozens of people like you and me, ready to open the wallet, waiting for so many years...
Good luck