Thanks a lot for your comments!

@kendog: I mostly agree. After all calculations the Antminer was my top 1 device. But even if I would get a used one for let's say 1500 USD, it doesn't look too good. Do you know the calculations from http://thegenesisblock.com/mining/ ?
I checked their calculation and it seems reasonable. The only thing I am not sue about is the projected difficulty increase - or the question if the dificulty increase from the last months can really be extrapolated into the future like they are doing...

In fact their projected difficulty increase is to steep that even Cointerra, Hashfast or Bitmine would not produce any ROI

@glendall:
1) sure, for litecoin might be an option.  Taking resale revenues into account makes the calculation look much better, especially compared to ASIC miners that may drop to a resale value of $0 quickly. But according to my calculations, the rentability of Litecoin GPU mining may become negative March or April (unless you don't pay for power consumption). Correct?

2) True

3) Would never do that. Especially not paying by bitcoins...

softworkz