What to Charge for Customization

I have a customer that wants to pay for customization of a product I sell. He won't get the source. What should I charge him?

It seems unfair to charge my full contracting rate for all the time it will take when he won't get the source code and I may benefit from the changes.

I'm torn. I want to be paid for my time and I would not be adding this feature if he did not want to pay for it. At first I thought I would charge him my full contracting rate to get the feature working well enough for him - but not charge him for: rolling it into the full release, documentation, backward compatibility, and other things he won't use. This is half the work. So, in a way, I would only charge half.

Then I think I should charge him much less because it may result in more sales down the road if others want this feature. I should be thrilled someone wants to pay me anything for improving MY product.

I have never charged for feature requests before if it was a good idea. But, this time I'm busy with other things and did not plan to upgrade the product for at least 6 months. So, any changes now, someone has to pay something. But, how much?

What would you do?

What would be fair?