Use your head.
If a POTUS spends $10 million on personal travel one year and then $2 million the next - that means that the government has to spend $8 million less dollars on the President's travel expenses or Air Force One costs or whatever the applicable cost(s) is.
Are you seriously suggesting that when a President incurs less travel expenses one year over the last one that the government, if it does exactly the same things otherwise, will still have exactly the same spending costs?
No offense - but you cannot be that dense.
Sure, they might say 'Hey, look extra money 'cause Obama didn't use it. Let's go get hookers for everyone and put it on his travel tab.'
But the point is, that money will be saved until otherwise spent on other, non-related things.