Bales statistics really aren't that amazing, in the context of this PL season ;
Mata 10G 9A
Walcott 11 8A
Hazard 7G 7A
Bale 13G 1A
Fellani 10G 3A
All players, playing in equal if not weaker teams, similar freedom and off similar ages. In terms of pure output Bale is joint 3rd with Hazard, from that list. Yes Bale is a very exciting player and a hot prospect but this 40-50mil stuff is just ott. He doesn't play like an old fashioned winger like Giggs did, he has the license to roam a bit, yeah he tracks well but he's allowed to drift a lot. I've no doubt that the likes of Nani and Kagawa at united could get a lot more goals in a more attacking role. That's not to say what he's doing isn't impressive, but in the role he's been given it's also expected.
That's the thing, Spurs have built their team around Bale and his goals have come from that. If he goes to a bigger team he's unlikely to get that freedom because you wouldn't say he has more ability/potential than Rooney/RVP at united, or mata/Hazard/silva/Aguero at chelsea/city, let alone the likes of Barca or Real.
If he kicks on and does follow in Ronaldo's footsteps in this more drifitng role then fair enough but that's a lot more goals he needs to find, if not I can't see how he's a 40mil purchase, when in recent seasons the likes of mata/carzola/silva have been got for closer to 25mil.