MSRP of a 1070 is $400, but due to the mining craze they generally retail for higher than that. 1070 Ti MSRP will be $429, placing it in between the 1070 and 1080 in terms of price (which is exactly where its performance will be too).
Miners will also most likely pick up all of the 1070 Ti's at MSRP until the price is up to (what my guess is) ~$460-500, which puts it right up there in price with the 1080.