In addition to the excellent advice already provided by others, I would urge you to bear the following in mind:

1. Software, like anything else built by man, requires maintenance, and hence, if they want to rely on it, they must plan and budget for maintenance, regardless of who will be performing it.
2. The difference in cost between a proof of concept written by an intern and maintainable, production quality software written to perform roughly the same task can be anywhere between 1% and 99%, and is usually much closer to the latter.

Thinking about maintenance as ''just this one more little thing and then it will be done'' doesn't make the cost go away, it just keeps them from planning and budgeting for it.

You might need to make this clear to them before accepting any further work. Don't phrase it like that, of course; that would be pedantic. But don't enter into any negotiations in which their position is based on ignoring or misunderstanding this.

In addition to the excellent advice already provided by others, I would urge you to bear the following in mind:

1. Software, like anything else built by man, requires maintenance, and hence, if they want to rely on it, they must plan and budget for maintenance, regardless of who will be performing it.
2. The difference in cost between a proof of concept written by an intern and maintainable, production quality software written to perform roughly the same task can be anywhere between 1% and 99%, and is usually much closer to the latter.

Thinking about maintenance as ''just this one more little thing'' doesn't make the cost go away, it just keeps them from planning and budgeting for it.

You might need to make this clear to them before accepting any further work. Don't phrase it like that, of course; that would be pedantic. But don't enter into any negotiations in which their position is based on ignoring or misunderstanding this.