He's the best...AROUND.. nothing's ever gonna keep him down
etc etc etc
As for the Fernando comparisson, they are probably about equal (though not the same, if that makes sense?) at the moment, but given that Fernando is a world class holding player at the peak of his powers and William is just a kid in his first season of proper first team football and improving rapidly all the time, it's pretty impressive.
As for if he's worth £30m+, that's obviously relative to how much money you have and how much you need him, compared to other positions. It would be a major investment, but if we have that money to spend on a holding midfielder for the next decade, then he's probably the stand out candidate. Personally I'd rather of had Matic for £22m or whatever it was, but that ship has sailed