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: 
I really can't answer for code....

... but ....

in my field, design, I'd find this comparable to illustrations. I customarily charge a multiplier for exclusive rights to illustrations. Somewhere around 3x or 4x works out well.

What I generally do is estimate the time needed to create the original... then apply the multiplier. This generally covers many future usage scenarios since that creation time would be needed at least once if I were to come across the need for something similar and *not* be able to use what I've already created or have on-hand.

This somewhat depends on **rights** as well and not merely exclusivity. If the client wants *all right* then a higher multiplier is warranted. If the client merely wants an exclusive license, then I negotiate a time frame for exclusivity. After all, if the client *doesn't* want the rights, but wants exclusivity - what benefit is there in me *not* transferring rights unless the exclusivity ends at some future point?

I *always* include the cost of exclusivity as a separate line item on quotes so that the customer can see what that specific factor is costing them.

So either:   

> 
> Exclusive License For XX years/months/etc ... $xxxx.xx
> 
> 
> 

or  

> 
> Transfer of Copyrights ... $xxxx.xx
> 
> 
> 

Some clients quickly change their mind about exclusivity when they see figures. On the other hand, like all quotes, some clients don't have a problem with the figures at all. 

In truth, it's very difficult to quantify values, at least for me. There's no telling *exactly* how much I'm giving up in terms of future usage. So really, it's all just "best guess" and figures are high enough where I won't feel I shorted myself when looking back on things.
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).