As said, if you are traveling, you aren’t doing other work. They buy your days, even if not full. More often then not, they are more full. “Social” dinner, is still working and talking shop, even if beers involved.
I charge day of my rate x 10 hours for each travel day, including days actually in travel (ie. in a plane, car, etc. that day is essentially a write off, unless it’s <4 hours travel). They pay for all expenses (food, hotel, airfare, etc.).
IT can indeed add up (ie. 14 days in China = $$$$), but you do get a lot done quickly you wouldn’t otherwise. You are paying for experience and problem solving and the costs are much cheaper to avoid the mistakes then to make them and have to fix later.
R