If you have to make a price for one-off customer, you end up having to support that use case as long as they are a customer. You may need to change your entire

If you have to make a price for one-off customer, you end up having to support that use case as long as they are a customer. You may need to change your entire business model just to support them. If they leave, all that change is worth nothing. Unless you're in the business of doing custom work, it's unlikely to be worth the opportunity cost elsewhere. After a certain point it's not about money. Your time and attention is worth more than they would ever be willing to pay. If the customer wants it that badly, they can pay the price to get it from someone else.