Attractive price for used AMD GPUs

[monsanto]

Assuming the card is bought within last 6-9 months, at what price would you guys stock up the used AMD GPUs.

Here it is for me:

Rx 470/480 - $80
Rx 570/580 - $90

Above is including shipping.

Wow, I didn't realize they might get that cheap. How much more efficient are these than the R9 290x? Are they also more powerful in terms of H/s? I'm trying to think of some uses for old GPUs. Or maybe I'll just give up and get a 1080ti