For reference here is information about previous releases of Nvidia:
GTX 780 Ti - $699 - (7.11.2013)
GTX 980 - $550 (18.9.2014) - 7% faster than 780 Ti, $150 cheaper, 10 months after the release of 780 Ti
GTX 980 Ti - $650 (1.6.2015) - 30% faster than 780 Ti, $50 cheaper, 18 months after the release of 780 Ti
GTX 1080 - $600 launch (27.5.2016) , cut to $500 (1.3.2017) - 27% faster than 980 Ti, $150 cheaper, 12 months after the release of 980 Ti
GTX 1080 Ti - $700 (5.3.2017) - 43% faster than 980 Ti, $50 more expensive, 21 months after the release of 980 Ti
(EDIT:
Anandtech puts 1080 Ti at +74% at 4K and +68% at 1440p over the 980 Ti)
RTX 2080 - $700 (20.9.2018) - 8% faster than 1080 Ti, same price, 18 months after the release of 1080 Ti
RTX 2080 Ti - $1000 (27.9.2018) - 29% faster than 1080 Ti, $300 more expensive, 18 months after the release of 1080 Ti
RTX 3080 - $??? (9.2020?) - 20% faster than 2080 Ti (?), unknown price, almost 24 months after the release of 2080 Ti
RTX 3080 Ti - $??? (9.2020?) - 35%-40% faster than 2080 Ti (?), unknown price, almost 24 months after the release of 2080 Ti
Performance figures are from techpowerup.com