How about buying those RX580 for 70-80$ each? Yes if this happens I will buy eight pieces to mine ravencoin only, the graphic card is very old but yes mining software still supports the graphic cards, we don't know what will happen in future.
It's hard to say if 70 or 80 USD are good prices, but if you have cheap electricity, maybe it's worth
Just do the math
Here in my country for example, my electricity rate is .20, so it's barely profitable, like 10cents per day per card (rx 580), putting 5 in a rig will be .50 cents a day = 30 USD month
Not worth the time and noise in my opinion
But let's say you have 0.05 rate, things start to look better
After ETH POS we'll have a better understanding about which cards will continue to mine in profit (if), but I'm doubt about Polaris generation. I suspect only RTX 3XXX and AMD 6XXX will continue on the game