Most travel management programs force employees to get a pre-trip approval from their respective line managers. This approval though is only limited to necessity to travel and not the cost associated with it.
It's absolutely important that approvers are made aware of 100% information prior to approvals. This will make the travel program more operationally efficient and also ensure that approvers are checking the travel requests sent to them by their reportees.
It's also important to quantify the cost of approval through a mechanism built in to the travel management solution.