I saw this train wreck coming last year and jumped on my EVGA 1080 Ti for $750 before prices got insane. I also took advantage of dumping one of my GTX 970s in SLI for more than they were worth this time last year ($225 vs. $175). My GTX 680 is worth $150+ now (eBay selling prices) when last year I'd be lucky to get $80 for it.
So all is not bad for those out there with older GPUs because the used value increase can offset, at least partially, the price spike of new GPUs. But I would not buy a new GPU right now as Nvidia's Volta line is coming out in July starting with the GTX 1180 or 2080 (however they will name the new series).
With that said, shop around. Prices are coming down (not for the 1080 Ti however). Last week B&H Photo had a GTX 1070 Ti for $530. It sold out in no time. We are dealing with a perfect storm crisis of epic proportion which has never been seen before, not even during the Bitcoin mining craze of 2011-2012 causing a rush on GPUs and price spikes and inventory shortages.
Besides miners, there's a strong demand on NAND memory modules from a massive spike in global smart phone demand (China the biggest culprit). This is putting challenges on production capacity competition between GDDR5 memory for GPUs and DDR4 memory for smart phones (a reason PC/laptop DDR4 memory prices are through the roof). Further adding fuel to that fire was a power outage at one of Samsung's fabrication plants. That resulted in 11% of Samsung’s monthly production being scrapped.