All work that an expert does for a case is billable, including travel time. However, experts will frequently provide discounted or even unbilled work for individuals in certain circumstances (like criminal cases where the expert is testing in a forensic capacity to counter improper forensic analysis presented by a prosecution expert).
Some experts charge for their time at a reduced rate (e.g. 50%) for travel, some a predetermined amount (taking the risk of delays on themselves), some only for the cost of the tickets, hotels, meals, etc.
There is, AFAIK and based on what I can Google, no universal answer.
