There was I thinking that £350 for a used GTX1080 card was cheap.
Seems that folks using these for Bitcoin mining realized early on that custom hardware was better, however the cards themselves have many other uses such as medical imaging, machine learning, data analysis, password bruteforcing, etc.
Serious gamers moved on a long time ago but for certain applications (eg machine learning) an array of cheap used cards on budget boards (eg quad core AM3+) will still do more work than one £xpen$ive card when suitably retrofitted with better cooling and updated thermal management such as heat pipes just for the RAM not to mention the latest software custom designed to squeeze every last IOPS out of the overclocked GPUs.
I once did this to my not-2-day-old RS480 and got about a 11% performance boost with no overheating.
Some other folks are leveraging used laptop boards with on-chip GPUs as they are quite compact but this approach is significantly more complicated due to limitations in clock speeds with a laptop.
Even replacing the base clock does not help here as the limit is in the interconnects.