Martinez isn't a striker going into Premier League. Tho he was largely overpaid, but that's today's market for you.
It's not like Dortmund is the only German team whose stars were exploited in the latest years. Ozil and Khedira for instance are now worth 2-3 times the fee they were sold from their respective clubs after World Cup. Sure, it's more visible with Dortmund but in general it just seems that Germans became a little careless in the market making some deals that could be grinded a bit more, but then again their league is most stable so I don't blame them. But I'm not the most knowledgeable about their financial state so maybe someone will fill in on that.
If you have the guys that are talented, you should be able to convince them that your team has the ability to establish themselves in Europe - that's the only way you will not have to enter negotiations and try to grind deals for young talents. You're never going to manage to get a lot from a transfer of a 21-yr old World Cup wonderkid like Ozil was, because no one, not even Madrid, are silly enough to take such gamble. That's why Sahin went for so cheap and that's why 22mln is a good price to take for Lewandowski (unless of course a certain Russian billionaire comes in and says he'll give you the equivalent amount his yacht is worth for the player
).
Bayern managed to keep Lahm and Schweinsteiger, Borussia seem to be holding onto Hummels and others. It's not all lost even if Lewandowski leaves.
Especially with strength of English clubs and the likes Real Madrid or Milan under question. Despite Messi being the favorite to trounce every team in Europe, if Borussia win it they'll become a more likely destination for all the talent. That is, if they can keep their fundamental boys.