Grading and a price

How does the grade (XF, VF, F etc.) affect the price? EXAMPLE: I've got two coins. (1.) - UNC coin for let's say 100 USD. The same (2.) coin in VF grade. How to tell the price for a lower / higher grade coins when we know the price for one particular grade (in this example for VF one)? Is there something like percentual range between particular grades or rules? How to deduce it?

Or if I have a coin with a clear grade and a price, is the price for all coins in this current grade or for a particular one?? 

The price is what somebody is ready to pay for…

We have to realize that there is no easy relation between condition and price, which is appropriate for all coins. It depends of the distribution of grades: For example a circulation coin with a long period of use (german empire, 2 Mark 187..) could have a price for au, which is factor 20 compared to the vf price.

If we have coins with a short period of use (portuguese silver 1914-1917), then the au coins are not unusual, their price could be factor 2 or 3 compared to vf price.

And this was said already, every price is fantasy, until it is paid.

Both answers are helpful. 

Even for coins which should be exactly the same value because they receive the same numeric grade (let's say NGC MS61), you will find a range of selling prices because every coin has different imperfections, luster, etc., or one was photograhed less professionally, or maybe two collectors both got a bit crazy wanting the same coin, and  bid the price high.

For common coins, do your research on auction platforms like eBay, then set your price.

For rarer coins, do your research on places like CoinArchives.

The 1- 70 grading scale we use today is derived from the “Sheldon Scale.” Dr. Sheldon originally published it in his book “Early American Cents,” and it was intended to show both grade and value. Thus, a Mint State-60 coin was assigned a value of 60 times that of a Poor-01, 20 times the value of an AG-03, 5 times the value of a Fine-12, etc. After the scale was adapted for use on all coins, it just became a grading scale, and today if you look at price guide values vice grade, you'll usually find the value goes up exponentially with a linear rise in numeric grade.