One eBay I have been looking at Radeon and GeForce cards at auction mostly to get a feel for market valuations. With the 4K panel mainstream cards are hard pressed to handle a wide range of games.
GP106 spans the GTX 1050, GTX 1050 Ti and GTX 1060. My existing GTX 1060 is still very respectable but the lack of VRAM has been the cards biggest shortcoming. It’s clear that nVidia chose to go cheap on GDDR5 chips rather than give all cards 6GB of memory.
GP104 span the GTX 1070 to the GTX 1080. Many such cards are available which means an improvement is not so hard to swallow.
GP102 spans the GTX 1080 Ti, Titan X Pascal and Titan XP. Such cards are extremely expensive and few have been seen on eBay for sale.
The new Radeon VII ($699 MSRP) has undoubtedly upset the valuations of the TU102 cards such as the RTX 2080 Ti and Titan RTX. The Radeon VII is between the Vega 56 ($399 MSRP) and Vega 64 ($499 MSRP) for the number of CU available but it offers double the VRAM at 16GB.
The GTX 1080 Ti still fetches over $400 at auction but the card is still well regarded. GTX 1070 cards are now down to $200 or so. So moving into an 8GB card is not too problematic.
The RX 570 8GB are now routinely under $100. The RX 580 8GB ($229 MSRP) are still $100 or so plus shipping. Even the Vega 56 ($399 MSRP) is now hardly $250 at auction. AMD has offered more VRAM than nVidia for several years now and even their mainstream cards they generally have more VRAM.