Looks like Gigabyte G1 980Ti's are still holding at apprx $375 USD... not bad but thought prices would have dropped a bit more. Just goes to show you what the public thinks about Maxwell (it's still very much relevant).
Anyone that already has a 980Ti, adding a 2nd in SLI isn't a bad idea.. It will handle 4K no problem.
Much cheaper than buying a 1080Ti anyway...
SLI support is absent is some titles but overall I've never really had any issues with modern games except the latest Wallenstein wouldn't run in SLI no matter what.