LOL, 15mln for goes for Matri, Pazzini, Ramirez.. something less for Borini and million of others. You must be joking.
actually, I am not
attacking players tend to go for more moneys than relatively unknown defensive mids/CB wanna be's ... and I am pretty sure that 70% of people here couldn't even tell you who JM until today and with good reason - he is much closer to being one of many good players out there as opposed to a difference-maker.
btw, I also wouldn't pay 15+ mil for those you mentioned but naming a bunch of overpriced/overhyped "talents" doesn't exactly make JM worth 30+ mil and make a deal like the one mentioned above a good one, does it? Let's not shift the focus from the obvious - JM is neither a great (defensive) CM nor a top CB, after all ... he is a good player, with good versatility in the deff. CM/CB roles but that just it.
anyways, price of a player is a subjective matter - Bilbao may ask for 100m for all I care. My main point, however, still stands - Bayern could invest less than 40 mil and get a CM and a CB at least as good if not better than JM, which won't be hard at all to find.