Digital copy...meaning your client wants only a copy to use. If the client wanted exclusive rights, meaning that only they can use it, it would most likely be a higher price.
Ok...a phone app and website, for how long? In perpetuity? I.e. forever?
This is the thing about pricing licensing, there aren't hard and fast numbers. A good price is one that you and your client are satisfied with.
I often start negotiations with asking what the budget range is for licensing the art. It establishes a baseline. From there, you can decide if it's too low for your comfort zone or if it seems reasonable.
Project confidence. Be professional by letting them know you appreciate their interest, and look forward to coming to an agreement.
A number you are both satisfied with is the goal.
Good luck!