The agents are paid the same regardless of their location. Yes, there are other costs involved due to travel; however, their salary is the same.
I think you missed my point.
When the SS are at the White House, it costs taxpayers nothing for them to go home at night. It costs no extra dollars to guard the family while they are on the premises. It costs no extra tax dollars to transport SS around, because presumably they drive themselves home.
It's added cost every time they go jaunting around the globe. They get paid the same, but taxpayers pay more than just their salary.
It's like saying war doesn't cost anything, because soldiers get paid anyway. There's more cost than just salary.