Conning people doesn't seem like a good way to get your message through to them.
Considering that exiting the right-wing circle often involves ditching most of your friends, someone to talk is the most important asset "on the other side".
- They had a way to reach their 'targets' directly, in private
- They wanted to establish a name. The German explanation sounds like they're trying to build up a reputation first
So yes, they were/are aware that these shirts won't convert anyone and they certainly expect no quick result anyway.
From what I read that "attack" was a big talk in the German right wing forums websites... => people now know the Exit program exists => so it worked VERY well.
Forget for a moment that we're talking about people whose beliefs we (presumably) strongly disagree with and imagine another scenario - say youth at a Democrats rally are given T-Shirts with Obama on them, take them home and find that after a wash they have a pro-T-Party slogan. I would think that pretty unlikely to change anyone's mind. They're more likely to be pissed off.
I can imagine the Exit people watching the concert and giggling at their clever prank. Cute prank, amusing, but as a technique for persuasion?
On the other hand: I know _I_ would've giggled nevertheless, seeing people walk away with those.. That doesn't change the fact that the whole thing was a _success_ measured against the goals of the creators.
The goal isn't to persuade or change peoples minds, but give people who have, on some level, decided that they want to leave the push and support they need to actually take the final step.
You can't be serious. What "threat of retaliatory violence"?
And to answer your question, approximately none.
It's not a con, it's a prank.