no clue -- i was an officer in the Minox Historical Society for a coupla years, and at that time -- 5 or so years ago -- B models were a glut on the market, $50 if you were lucky. The black ones bring more. The newer models with electronic shutters bring more, but even those are relative deals.
So why are they suddenly desirable? No clue -- I wouldn't pay $200 for one in mint condition, and the meters are usually (a) dead and (b) impossible to repair at a reasonable cost point. The film is very expensive unless you cut and load your own.
And, let's be honest, they are a lot more work to get good prints from. I go on a minox binge once a year, frustrate myself for a coupla weeks, and then do some 35 and suddenly 35 feels like large format.