What are you basing your valuation on, Zimm?
The money being paid by other teams all around the market, in this scenario Newcastle don't really want to sell (and certainly won't want to so late in the window), he's one of their best players so clearly valuable to them, what does £10 million buy you in the market these days?
To replace him they'd basically have to go in and buy someone untried at this level if they sold him for £10 million, for 15-20 million they can go after more proven players however.
Chelsea paid 18 million for Schurle (9 million for Van Ginkel a guy from a small club who's not that well known), Wanyama cost 12 million from Celtic (their players go cheap), Paulinho was 17 million, Caroll was 15 million, even unknowns go for 8 million, on that basic Cabaye has got to be worth 15 or so.
However with it being so late in the transfer window the price is probably a bit higher now.