We just finished liquidating one of our mines a few months ago, so things probably haven't changed that much.  Our mine was considerably larger than yours (about 4 or 5x), and was exclusively Nvidia GPU's - but a mix from 1060's to 1080TI's... We sold to one buyer, primarily because we didn't want to go through all the pain and suffering of selling the individual parts - and he bought everything; cards, cases, PDU's, cables, you name it.  For the cards, what he was paying was basically about 30% off of what eBay sells for - so just a quick check looks like RX580's go for around ~$120 range, so if you have the boxes for the cards, figure probably ~$50k, if you don't have the boxes, then maybe closer to $40k (he knocked off about $10 for no box, but the bulk of ours had boxes).  He also paid more or less for different manufacturers, as some have far better warranties than other.  The cables and everything else are really worth very little - we bought most of our PDU's and cables surplus, so we already were getting the $500 Dell PDU for $50, he basically paid us about half of that, so $25 each.  Really the bulk of your values is going to be in cards, so my guess would be closer to $50k for everything you've got - really I'd be surprised if a miner would buy it, you're really talking about a liquidator, so I don't think you'll find you have any 'in place' value (especially as others pointed out in terms of ROI).  We also ran our systems very similar to yours, custom Ubuntu although we had local SSD's on each machine (as they were cheap as hell) - but he didn't really care one way or the other about that, just interested in hardware.  And keep in mind this was for Nvidia GPU's, so I have no idea if the market is stronger or weaker for ATI...