I'm just starting my research into my future first mining rig (I'm planning on using ethminer through nanopool btw). I have done a ton of research and have done some tests myself, and it seems that all of the 470/480/570/580 cards are taken up and sell at least for $500 USD. However, I did a lot of searching on Ebay and my local Craigslist and I found some sweet deals.
There are many auctions on used MSI Twin Frozr R9 270x used 2 GB cards with the highest bids currently going for around $60-$80 USD. This seems like a good deal when compared to a brand new RX 550/560, but I'm afraid that 2 GB cards are simply no longer profitable anymore, since there are way more people using 4 GB or 8 GB cards now. Currently, all of the cards going on for auction have not been used for mining of any sort. If 270x cards are still profitable (I checked Ethereum calculator and here are the results:
There are also some 280x 3 GB cards on auction with the highest bids being ~$120 USD. I checked the description for each of them and it seems that none of them have been used for any mining, only gaming. There was also another one that is on sale on my local Craigslist (specifically a Gigabyte Windforce) 280x 3 GB for $160, and if the 280x is more profitable than the 270x in the long term, I might consider buying it for asking price.
It's likely that I'm probably going to have a mix of 270x/280x cards, since 290x cards are going upwards of $230 on Ebay, and considering that the 280x cards are selling for ~$100 less, the extra profit isn't going to matter.
But I hope that at the very least 270x/280x cards will be able to hold up for the next 3 years or so.