Off course, Uber take the return fee of your lost item. Uber take this fee as the compensation because driver make some time for you when you coordinate with him to return the lost item. Uber drivers have their driving schedule and they make time from it.
It was your fault that you lost your item in the car. Uber don’t take any responsibility of your item. You lost it and they are returning you. A proper returning fee is applicable in such cases. Fee is decided by Uber in every country where Uber service is running.