Okay, then say it this way: Since he left Leverkusen 2013 he hasn't performed the way that his price tags are justified. He's now Dortmunds most expensive buy ever and so far only Bayern and Wolfsburg paid more for a player. And that for a guy who has one good half year in the last three seasons.
If he doesn't find his Leverkusen form his price tag isn't justified because and I don't consider him as a good buy or a reinforcement for Dortmund after the sells.
Look at the price tags for this summers window. Is Higuain worth €93M? Pogba €120M? The prices are at the moment way out of reasoning.
We are talking about a 25 yo international German footballer with 55 caps and who did alright last season. €30M is a great price when looking at the price tags for other players on the market. You don't get 55 caps for a successful and talented German national team if you aren't performing well over a longer period.
These are the most expensive players bought by Bayern München according to Transfermarkt:
Javi Martínez (40,00 Mill. €)
Mats Hummels (38,00 Mill. €)
Arturo Vidal (37,00 Mill. €)
Mario Götze (37,00 Mill. €)
Renato Sanches (35,00 Mill. €)
Douglas Costa (30,00 Mill. €)
Manuel Neuer (30,00 Mill. €)
Mario Gómez (30,00 Mill. €)
He would have been tied with Costa, Neuer and Gomez if they had bought him and not as their second most expensive player like you wrote.