>Is it common practice to charge for extra copies to someone who has payed for the original development?, if not, why? just as a good bussines practice or is it plain out unethical?

If there was original language in the contract that specified that the client owns the work, then I don't think you have a legal basis for charging for additional copies. If there a legal clause that the work was done on a "for hire" basis, then you do not have control over the end product

As a practical matter, if you think this may be an issue in the future, then you should try to get an agreement before development. In cases like this, I would use a clause that says something like "the system is designed for up to 10 simultaneous users within a single building connected to a single file server. Exceeding this number is requires an additional per-user license..."

It is not unethical when you consider that the client originally payed $X for a certain value, and now by extending the scope of use, they are receiving an additional $Y of value. The question is, how can *you* relalize a reasonable return on their incrase of $Y.

I have a situation now, where an agency wants to sell one of my applications to another agency. They don't want to make a lot of money on the deal, but they would like to recover some of their development costs. Because I'm personally on very good terms with both agencies, it is probably in my interest to allow this transaction to take place, because the extra work that I will get from the 2nd agency will be substantial, and yet, they wouldn't have necessarily bought an entirely new system from me.

Your mileage may vary. :-)