The reports are saying over 40 million which is a lot of money. Ozil money in fact. I know they don't need it, but turning it down makes Mata a very expensive bench player.
To be honest, Manure are just going to spend 40 million on some other top midfielder if not Mata in the summer. In fact they may well sign Mata for a huge fee and still make 1 or 2 further massive signings in the summer.