Guys, thank you all for your comments.
Yes, with my poor hardware you are right saying: Better don't touch it. With the Ethereum charts bouncing high and low- It was roughly 300€ in June, 150 in July and back to 300 in August- using a cost calculator is quite useless. And anyway: It only would be a few bucks/month. But I think bitcoin mining wasn't so much fun back in the days either, right? I'd like to see it as a bet that the other crypto-currencies evolve eventually like bitcoin.
But you're probably right: it's not worth it.
Damn, and I got no big rendering project on the horizon and I love heating through rendering so much in the wintertime :D