Its not a question of price. With friends we want to see the first months profits, and then invest BIG. So in 1 month i'm pretty sure it will not be late to get Nvidia's card for Zcash or Zpool.
The opportunity to get 20x RX570 is not offered to everyone so I think it would be a mistake to not take them.
It is not an insane purchase, you just need to make sure that you get everything that you can out of it before you hit the wall. There will be a point in time when this is not going to cut it anymore. When the ROI starts to drop, you can try switching down the line to a few coins that are not being nailed to the wall and that should buy you a few months and after that, it is simply time to upgrade.