A lot of time, it becomes necessary to separate some section commonly used like banner, footer to a separate file and later on include this parts as $this -> render('banner.phtml').
When this is used extensively, how costly does it become performance wise?
I don't think that it has some sort of bad effect on performace. I am working on very large product and I have extensively used $this->render() in my project for handling various things. If you are not having a common layout then you have to rely on this function. But if you are having a common layout throughout the project, then there is no need of this. So don't worry on using the render function.