Factors to consider and the obvious ones have been stated already, but let's place in some others...
A winning team
A title winning side
Champions League football
New Stadium soon
Nike deal
The chance to play with elite players like himself
Not having huge upheaval of moving
Playing for one of the worlds best managers
There is just some of the reasons this is very possible, more than possible because I expect us to not even bat an eye lid on 40-50M for him. If he wants it we need to make it happen, he's absolute quality and deserves to be playing football for a top side, and not a side that is a team full of average players with no direction.