What is the price difference between a auto focus and non auto focus same lenses?

As nikon 3100 don't have built in auto focus motor so i need to think is it worthy to buy a cheap body of nikon 3100 and later spend more in lenses.

All dslr cameras do auto focus whether it's from the body or, as in Canon's case, a motor in the lens.
You're not reading the fine print. There are no dslr lenses that don't do auto focus… That's in film cameras… At least many of them.

"Cheap" doesn't even come into it unless $700 or so is cheap to you. Any good lens is going to be at least $400.

It depends. Some non-AF lenses are cheaper than AF lenses such as in the case you are presenting. Some manual focus lenses are way more expensive than even the most advanced AF lenses though. That's because of their superior glass quality.

With Nikon, if you get a cheap body, you will have to buy more expensive lenses.