How much does the iPhone 4s, iPhone 5s, iPhone 6, and iPhone 6s cost per month with AT&T on contract?

All depend on what plan options you take when buying the phone & how you use it in a month.

Probably on their site, not sure why you expect people to cut and paste it for you

you could check their website. last time I looked it was more than purchasing the phone
http://www.att.com/cellphones/iphone/iphone-6s.html?&WT.srch=1&source=ECPS0000000PSM00P
You may want to see what promotions that Target, Best Buy, Apple Store, and Costco have. I don't think you will find a new iPhone 4s but you might find a 5c on closeout.