There is absolutely no point in going for the 20-series cards for use with DAZ - as they're not compatible, and probably won't be for at least half a year, if not more. (Note: DAZ only very recently updated their Iray to be compatible with Volta, and that is only available in the 4.11 beta version).
So, you can get a decent render-card (1080 Ti) for about $500-600 (depending on where you're located, of course) or a slightly lesser card (like the 1080) for a bit less.
Of course, even the 1080Ti only have 11 GB VRAM, which... isn't always good enough, and if you want more, you'll really need to spend 🙁