I guess the wage could be a big factor here.
I don't believe however Gotze could switch for only wages. He could have waited a year or even two. IMO, it's the Pep factor, false 9, the NT and '14 WC that have had a combined effect on his switch. Also, being a Bavarian I guess he might have thought of it during his kid days.
Though I believe, Lewa could be inspired by money.
The problem is in Dortmund Gotze, Lewa, Reus and Hummels probably are the highest paid and they get around 5-6m a year. Correct me if I'm wrong.
If you think logically there are several players who earn more than that in other places and he is probably thinking to move along now for the raise. He's 24. If he extends for 3-4 years, then there would be no sense in moving when he'll be 28. If he moves on now he'll probably be offered around 10m a year. So there's that obvious factor. I am not sure Watzke or Zorc would agree to pay such a huge amount to a single player annually.
Sahin moved due to the Real Madrid attraction plus Mou. It's the most attractive destination for players. It always has been. If Real comes calling, players leave their clubs.
Kagawa's was a case of exposure and fame. I remember reading an interview where Klopp said that Kagawa is moving because it has been a childhood dream of his. Well, only because PL has had an excellent exposure in Asia and people in Japan identified ManUre more than Dortmund.
Gundogan too might leave but maybe 1-2 years later. With RM, Barca and MU all keeping tabs on him. He'll fit in any where.
The EPL and Real fascination has almost always worked. So does Barca now.