It's not at all unusual for a service business to require an up-front deposit for work to be performed on an item that's left in the shop, especially when the value of the work is close to, or even exceeds, the value of the item when it's brought in.
It's a form of insurance in case a customer refuses to pay after the work is performed, or, for whatever reason, never collects the item. It's also a demonstration that the customer has the ability to pay, which isn't always the case. I spent 20+ years working in such a business, and ran my own for half that time. I only remember one occasion when a prospective customer refused to pay a deposit; it was much more the norm for customers to ask me up front how much of a deposit I needed.