Prestige Travel Services would be something to look into. They have a large amount of software and personal options for creating budgets and managing travel.
From my experience, creating a travel budget is unique to each company based on expected needs, culture, and resources. If you shoot me an email, I would be happy to share our Business Travel policy which may give you some basic principles when undertaking the task. Phil Brunner firstname.lastname@example.org
Travel policies don't have to be complex - they simply need to address who can travel, when, for what reason, how, and what the company will reasonably reimburse. That policy will most likely be different depending on the role (i.e. an account manager should have a client budget which includes their travel, or a consultant has the ability to bill their travel back to the client). It might also have to dovetail with the entertainment policy. Are they looking to save because they feel that they're overpaying for travel, or that they simply want to reduce costs? If it's the latter, they also need to determine whether any savings result in increased administrative load, reduced client experience, or employees who feel over-managed.