Sorry, let me try to make that clearer. So I invoice my clients for sum S, which consists of my travel expenses ("E") and my fee for actual work done ("W"). And I've had a couple of trips lately where E has been almost as large as W.

This is actually what I'm getting at -- surely there is a connection? At the end of the day, what matters to the customer is their total cost S (and that the work gets done!), not how it's divided up between my expenses and my profits.

An illustration: Let's say there's a local guy who charges W per day with no travel expenses (E=0). Why would the client be willing to instead pay me W+E, or twice as much? Because they feel that they're getting over twice the value out of me.