As most probably know already, the surging value of Bitcoin has driven GPU prices into the stratosphere. The RX580 is a great Mac 4,1/5,1 card, but sadly is also a great mining card. Once AMD stopped making new ones, prices skyrocketed. We continue to get a few in, but at current prices they are pretty expensive. With any luck prices will return to normal some day soon.
Meanwhile, now is a GREAT time to get your existing cards flashed. With greatly reduced card sales, we have more time for flash jobs. Most are in and out in 1-2 days. Please test the card first, flashing doesn't fix dead cards.