As best I can tell, the federal travel budget is about $9 billion.
There are roughly 2 million federal employees, not counting the military (1.5 million), military reserves (884,000) and people whose livelihood relies on federal government contracts, grants or mandates (another 12.7 million, directly and indirectly).
http://govexec.com/features/0199/0199s1.htm
That works out to $4,500 a year per employee. Between airfare, lodging, food and incidental expenses, you could hit that total with 3 or 4 trips.
Not everyone would travel that much, of course. But most people would travel at least a bit -- to training seminars or conventions, if nothing else -- and some would travel quite a lot.
Your point about "is it necessary?" is a good one that should be pursued. But in the scheme of things the savings would be relatively tiny.