Question: I am negotiating a contract where the customer wants an exclusiveness option. The project is about development of software and exclusiveness means that I am not allowed to resell it to third parties later.

What coefficient should I apply to value this exclusiveness in the contract ?

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It is worth to know that this piece of software is indeed reusable by others as such or with minor adaptations.

Answer: 
Although I have never signed contract for the purpose you state, a friend of my how is a game developer and has his team, does this often. 

They charge their time for one price and as the output give you ONLY the game. A client does not get access to the source code. 

If a client want the source code, they treat this as if they have built some generic platform which can produce more games with a different theme and charge 5 times more. 

You may try to think about a similar way for charging the exclusiveness they seek.
You won't find a coefficient that you can apply, as there are many factors to consider:

* How much work is involved?
* Do you charge per hour or fixed price for the software?
* Can you reuse some of the code in other applications?
* Will you also be given the same exclusiveness regarding maintenance of the application? (i.e. the guarantee that you and you alone can do the maintenance)
* Is this a product with an annual license fee or do they just pay for the software and that's it?

In short: if you just make the software, no license fees, no maintenance,fixed price, and you can't reuse any of the code in other applications I'd be putting a very big premium on the total price. 

But it really depends. In the end you'll just have to crunch the numbers and decide on a price that makes it worth the effort for you. 
I would recommend to add some recurrent revenue to the contract (e.g. maintenance or yearly license fee).