Considering the RTX 2080 only performs about the same as a GTX 1080 Ti on average, and costs $800+ at this time, I would not expect the price of the 1080 Ti to drop significantly any time soon. The 2080 also has RTX features, which might make it more desirable in the future, but as of now, no games support that functionality whatsoever, so I can't see all that many people dropping their 1080 Ti for a 20-series card.