How much will an iPhone 6s Plus cost?

The last iPhone 6+ (the 32GB models) was discontinued over 2 years ago now. Apple has not made, stocked or sold iPhone 6+ models in a long time now. So the only ones you’re going to find are used devices. And they cannot be upgraded beyond iOS 12.5.3, so there is an ever dwindling supply of apps that will run in them.

It is not worth buying such an old device as you will find that much software, and ever increasing more, will not run on them as many Apps require iOS newer then iOS 12.

The iPhone 6 Plus has not been sold by Apple for almost 5 years, so your chances of finding one are minimal.