Never tell people how long something takes you to do.
Instead, tell them how much the product is worth and what they need to pay for it. Just because something takes you twenty minutes to create, doesn’t mean you need to charge them for twenty minutes only. More than likely it took you a short amount of time because you acquired enough skill or have enough talent to have produced it in such a short space of time. Remember that cheeky retort from Picasso?
You work to value yourself not the client. Your only responsibility in that transaction is that you live up to your promise of delivering the product on time and to their specifications. You decide what they will pay for it and how they should interpret its value.