How do you compose the price for the end product if it needs more then usual amount of retouching?

If you shoot a session, where the end product will have much exposed skin (face/body), how do you compose the price for the end product? Do you have fixed prices? Or do you calculate retouching separately from the whole post-processing?

I’m new in this business and would very like to know opinions of those who makes photos for life.

A variant appearing to me as logical, is:

• client pays for the shooting on the hour basis, like hours amount X hour price,
• client pays pays for the end product depending on how many images should be delivered, like amount of delivered images X average price for one completely retouched and post-processed image, because of retouching of each image could take different amount of time.

So the price formula would be like b hours X c $ (for shooting) + m images X n $ (for retouching/post-processing).