The threshold can't be determined to a particular number, because so much of it depends on your facts and circumstances, and what you show (and tell) CBP.
And it's worse than that. It casts shade on the purpose for your visit, risking a refusal.
What I'm about to say is unfair. For many of our HNQ visitors, it's shocking that the 4th and 5th Amendments don't seem to apply at the immigration desk*. But I am answering OP's question. OP is not asking what fairness ought to be; we know that answer. OP is asking what is likely to happen.
You're a citizen of an advanced country that has a state-of-the-art banking system, competent and uncorrupted law enforcement, and is a member of G8 and all the relevant banking treaties. Not every German travels to the US, only certain ones: and the ones who visit for (your visit purpose here) tend to reside in a certain socioeconomic class. The vast majority of them simply use that banking system in the normal way. Which makes you a "high nail": a standout.
So this raises questions about you. Aside from the question of civil forfeiture, it raises the question of whether you should be admitted in the first place.
You would first, need a darned good reason for wanting to do this, that jibes with your story, believable documents, and allowable purposes for visiting the U.S. And second, have that reason be one that sounds perfectly reasonable to US CBP.
Remember how immigration control works. They make a presumption of immigrant intent: if you refuse to defend yourself, they presume your intent is immigration. That is reasonable, since their mission is controlling immigration by stopping immigrants, and only allowing in people who are short-term travelers. Then, they allow you to present your story and facts (documents) to prove yourself not an immigrant.
Your main weapon in rebutting immigrant intent is to show ties at home. One of the "ties" they like to see is an active, regular banking life. Having a pile of cash suggests the very opposite: a person who is "unbanked"**. Such people tend to overstay their original promise, get in trouble, seek social services or employment, or even resort to crime. Also, they tend to be poorly rooted back home, and that introduces the risk of them immigrating.
So the pile of cash will "light up" questions like this about your visit. Aside from having a reasonable reason for the cash pile, you should be well prepared to document your home ties back in Germany. Owning a home, being politically active, having a well-developed career that doesn't lend itself to telecommuting, that kind of thing.
You'd be sorting all this out with immigration control (CBP) right there at the immigration desk at JFK Airport, and if they refuse you, you'd cross an ocean twice for nothing. Here are two ways to dodge that.
- Don't rely on VWP; apply for a visa, and say right in it, "I know I am VWP eligible, I want a visa because I plan to bring $6000 cash for Very Good Reason Here, and I want to disclose and clear that in advance." (that is not something a criminal does). If they had a problem with that, they would hang up your visa or contact you. Immigration loves this kind of advance disclosure, not least because criminals don't do it.
- Book via Dublin or Shannon. Those airports have CBP Preclearance, which means the CBP desk is right there. If they refuse you, you can grab your bags, leave the airport, pick a four-leaf clover for next time, and head to the nearest pub to drown your sorrows in a 568ml Guinness.
* Because a traveler entering a US border is explicitly not protected from search and seizure. As to the right not to self-incriminate, yes, that applies in full and a visitor can plead the 5th at the immigration desk, and that will stop further questioning. However, she will also be refused entry, since she has no right to enter the US (that's what citizenship means, and certain other limited categories.)
** Unbanked: A concept we are VERY familiar with here in the US, and often taken as a bad sign. Again, within the demographic of ordinary travelers, this either means a very unusual philosophy on money, or (will be presumed to be) inability to bank, due to a blacklist arising from bouncing checks and not covering it, presumably because of not having the money.