At my club, when the subject comes up for discussion, it is assumed that the pilot must pay a proportion of the cost of the flight to stay legal. Free flying, I was told, is a no-no.
The assumption is generally correct. You can't go wrong by working under that assumption. However, the ANO, in article 160 and the surrounding articles, defines a number of specific exceptions. And this just happens to be one of them. See Legalapproaches post.
Short version is that the company can reimburse your direct flying costs provided that it is not aerial work (in other words: there have to be other means of reaching the destination, for instance making the journey by car or commercial air) and your employer cannot force anyone to attend the flight.
As for what "direct costs" are, well, it's a minefield, as IO540 pointed out. If you rent from a club, just submit the clubs bill plus any landing fee bills. But if it's your personal aircraft, you've got to find a way, if you want to, to include things like financing, depreciation, annuals etc into the "direct" costs of flying. For instance by going through an intermediairy company.