If I was selling something, I'd explain that your work costs more than the sum of used materials. It's your time, your efforts, your skill. Also, simply ask them "why do you want a discount?". If the answer is "the price is too high", maybe it's not your client, after all?
In your case, just tell them, that you don't have any discounts for now. But if they're going to buy something else in the future, you could consider it.