Raising request authorization for business trips without estimated amount

Summary:

Business Trip process: business user needs to go on a business trip, so needs to get approval to allow going for travel.

The normal business user has no idea what is the expected cost for this trip. They know they will receive per diem for payment of hotel and food. The airfare ticket will be arranged by travel department. So how can the business user raise a request authorization with an amount BEFORE any ticket is booked? The user raising the spend authorization does not know any amounts.

Question: How can the user raise a request authorization / spend authorization without estimated amounts.