The 16GB iPhone has been Apple's entry-level option for some time now, despite being more than a little controversial. Still, despite some rumors to the contrary, it looks like the company is sticking with it: 9to5Mac reports that the the iPhone 6S and 6S Plus will have the same storage options as the iPhone 6 and 6 Plus, starting at 16GB and moving up to 64GB and 128GB. On-contract pricing, the site also reported, will likewise stay the same as 2014 models: $199, $299, and $399 for each storage tier of the 6S, and $299, $399, and $499 for the 6S Plus.
http://www.theverge.com/2015/9/2/9248415/iphone-6s-16gb-storage
http://www.theverge.com/2015/9/2/9248415/iphone-6s-16gb-storage