There are expenses that will always be the same rate every time. Mileage is the most common type of expense where the billable rate or the reimbursement rate is the same and set across the Project Insight workspace.
Use Expense Code Types to set this rate across the entire workspace.
In Expense Codes administration, add the columns for Type and Unit Prices you plan to use (Actual vs Billable vs both):
When displaying those columns, you can set that the Code I've named "Mileage" is a mileage code and should reimburse at $0.545 every time it's used.
From there, the user selects the Expense code like normal, and Project Insight auto-populates the information.