There are 2 aspects to this:
1.) The board is incompetent. 15, or the even more ridiculous figure of 20 million, for a 31 year old defender, who isn't even selected for his national team, is just blasphemy. Only our board could even THINK of a deal so retarded. To put in perspective, felipe luis - currently one of the best LB's in the world if not the best right now, who i would have literally jizzed my pants over in anticipation of a transfer here, went to chelshit for 19 million. Just think about it...Oh and he's not even 29 yet...
2.) HOWEVER, I think the board is kind of in a tight spot, cause the clamour for defenders is growing, they have dug themselves into this stupidity thanks to their incompetence and overall retardedness, and with options flying out of the window, they feel they need to get Mathieu (not to mention that as the biggest club in the world we can't even get him without a transfer saga, it's quite embarrassing). But, they know they will get flayed for overpaying to the extend of 20 million for him. I don't think its about the 5 mn, its more about how they will answer to the directors/board/fans etc. at such shady dealings. That combined with the fact that any other signing is going to be more expensive because of the pathetic precedent 20 million for Mathieu would set..
But overall, I like him as a player, I think he'd be a good squad player, and any other club in the world would be paying around 5-6 mn for him..knowing barca i wouldn't even mind paying 10..15 or 20 is just mind numbingly bizarre, and one way or the other, this transfer is going to disappoint, be it us overpaying or not managing to get him at all.