>Thanks Gaylen, some food for thoughts. As you say, it is hard to work it out. The buyer wants the source code and wants to pay as little as possible. The program is unique, specially designed for them. I charged them $1000 two years ago.
>Thanks again.

I've never been in your position but, FWIW, here are what I think are the relevant factors.

To me it depends on why they want the source code.

If, as I think Gaylen is suggesting, it is to sell the package on their own perhaps you can work out some sort of sales commision rather than a straight purchase.

OTOH, since this is designed specially for them, my guess is that they want to maintain the system themselves rather than pay you for any fixes or upgrades. The price should in some way reflect your lost business from either a liscense fee or from the modifications.