I'll bet it's large but nothing close to that of the President and other top government officials. For example, the U.S. government is currently paying $2 million per month to protect former Secretary of State Mike Pompeo due to Iran threats[1]. That's the former, now think of what it'll cost to protect the current Secretary plus all the private travel costs on that giant Boeing 757.
It seems likely he gets less bang for his buck. The Secret Service is a much larger organization so they can take advantage of economies of scale. On the other hand, government agencies can be very inefficient when permitted to be, so who really knows.
1- https://www.pbs.org/newshour/politics/us-pays-2m-a-month-to-...