Cesc Fabregas is worth ________ to the club

I just saw (via the ol' twitter) that Barca are upping their bid to £31m for Cesc. Regardless of whether this is true or not, it raises that annoyingly seasonal question:

How much is Cesc worth to the club?

I was considering writing a bigger background for this bit, including all the special goals he's scored, the leadership qualities, the world-class skill, etc, that he has brought to this club.