i freely admit that I don't closely follow the German league. However if you remember last year when the DeBruyne struggle was taking place, there were reports mainly from Bayern saying things like "teams should not be selling their best players outside of the league as it doesn't help keep the league strong." Adding those comments to the fact that you just don't see a ton of big names leaving the Bundesliga, albeit with a few notable exceptions. It may have been lazy on my part to assume it their player policies were pretty incestuous, but there are reasons behind my incorrect assumptions.
Every year we see a ton of players move from Spain, France, and Italy to the Premier League. And maybe I'm incorrect on this as well, but it just doesn't seem like they have the exodus out of Germany like they do in the others I've mentioned. I guess Neuer was considered expensive at the time. By today's standards, 18 Million Euros seems like a pittance for a top keeper in his prime. Gomez was very expensive though.
Sorry for displaying my ignorance in such glorious fashion.