I think you also have to consider the relative difficulties, regardless of what gets spent by the club, it will be difficult to better the past two seasons at Southampton whilst staying there. He has to finish 5th or win a cup to better things (whilst no doubt losing key players again). That's with Europa football providing an extra drain on the squad.
Everton has no European drain, loads of money to spend, a squad that could probably already play at top 6 level with half decent management and less likelihood of selling off and all he has to do is finish top 8 to be seen as a big improvement on this season. He could go there and make himself look fantastic without even getting out of bed.