At Mozilla there were some meetings to discuss the financial cost of travel. There have been some follow up posts to discuss how to cut those costs for Mozilla as a whole. There are other costs associated with travel, other than financial that are just as important.
Flying contributes to climate change in a more damaging way than many other forms of travel. We could offset this with some carbon credits, but probably a much better bet is to not do the travel in the first place. Now that Mozilla has multiple offices, its worth considering if there is an office that is closer to everyone and hence reduces the air miles travelled.
It's not just the time for the flight, but all the waiting around at either end getting to the airport and so on. I'm lucky in that a trip to Mountain View is only a two hour flight, but add in a three hours to the airport and then another hour at the other end. Others who have to travel longer are less lucky with trips to some offices taking days.
As a result many work weeks or conferences factor in a travel day during the week. Some add in a travel day on the weekend, which is even worse for many employees well being. Travel cuts not only into work time, meaning less gets done, but into personal and family life as well.
Some people find travelling stressful, others find it makes them sick. Crossing time zones repeatedly isn't easy and wears the body down. It's pretty hard to function at full ability for a day or two if you've just travelled 8 time zones. There's a reason for that, its hard physically and mentally.
But bear in mind, it's not just the well being of the traveller, but also any family. People do not live in isolation, but have family to care after. A trip can cause stress on the family with arrangements having to made for dependents and others. I'm lucky in that I've got relatives nearby who are able to help out with our kids, but I do recognize the burden it puts them under every time.
At the end of the day, its about people and there's a cost to everything to you do. If you are a manager, then knowing the cost of travel and bearing this cost in mind is just as important as the financial costs.
Rarely on an exit interview, or in an interview for a job, will you hear people say "I travelled too little". It's more likely "I travelled too much". There's a reason for that.