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What did they say?
As far as pricing, how is this hosted/ran where is that website they download from - do you maintain it? Why only have one customer?
are you selling an "app" - then its a one-time fee.
What you really need to get from your single customer is really how much value and $ it provides him and he gets that discount because he is customer#1 and is sharing data with you on how well it does for him -- which would allow you to know what value it can provide to others...
If you cant find other customers, either its a one-time fee to him (proportionate to the value it provides) - or you need to do a monthly or per event cost.
If he is doing 5-15 shows per year, make the app $1,000 per show, and roll and explain your pricing to him.
Limit the number of hours/events that youre available for support.
Otherwise - move on.
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You made a mistake. He didnt ASK you to do anything - he PAID you to do a thing (unless you already built it for free)
Charge a Fton if he wants [feature] - else, you are either partners, or your an hourly consultant. based on your comment - it would appear that you SOLD him the entire thing already.
What you should do is say that [feature] will be in your next release, and he would have to subscribe to that new release to access [feature]
If he is reque$ting [feature] - then he should pay for [feature] - value your effort and time and consternation accordingly. (but in his mind, I think he feels he bought and paid for the whole app)