That depends from area/region to region. Some places have the GPU at horrible prices(if you're in select parts in Europe) while others have some amazing prices. Where are you located and how much does it cost where you've found it?
You may find it in your best interests to pass on your full system's specs like so:
CPU:
Motherboard:
Ram:
SSD/HDD:
GPU:
PSU:
Chassis:
OS:
In case your system can't accept the GPU you have in mind.
1070Ti is about 10-15% weaker than 1080
Depending on what do you have to pay, I can be a good deal.
The preises of 1070, 1070Ti and 1080 are quite near to each other and can overlap.
You need to really check for prices first.