Depends on what resolution you are running. Very few people actually NEED a 1080 Ti.
For example, I have a Zotac AMP! Extreme 1080 Ti, and I run games at 3440 x 1440 100Hz Ultra, no exceptions. There is exactly one game (barring unoptimized games that don't utilize the full CPU) that my avg fps is under 100, and that is Far Cry 5. And even that runs at 55fps at 4k 60Hz.
So unless you are running a very high resolution something less will work, or you might even wait for the 11 series.
[strike]Volta[/strike] (Actually they're saying its Turing now, not sure why they are going away from Volta seeing as the Titan V is the big boy on the block now but whatever) should be coming soon enough. I think latest estimates have it at Q3 2018 reasonably it should be about August- early September, however some sources say as early as July 2018.
But there is also the possibility it won't be released then (since an official release or announcement has not been made) or will be pushed back so it's really up to you.