I'd definitely hold out for £20m. Today's market is strange though - £20m is less than what Carroll, Downing (ish) and Illarramendi fetched (Yo is better than them) but more than what Cazorla, Coutinho, Navas and Ozil cost (who are all better than him). So there's not much perspective anymore. But with 3 years left on his deal, we can certainly demand a large fee. Maybe take note from Spurs when they held firm with Modric.