聯邦法 : 28 U.S. Code § 1821
A witness shall be paid an attendance fee of $40 per day for each day’s attendance. A witness shall also be paid the attendance fee for the time necessarily occupied in going to and returning from the place of attendance at the beginning and end of such attendance or at any time during such attendance.
州法,看所在地的相關規定 (這是一˙個例子 加州的法律)
Except as otherwise provided by law, witness' fees for each day's actual attendance, when legally required to attend a civil action or proceeding in the superior courts, are thirty-five dollars ($35) a day and mileage actually traveled, both ways, twenty cents ($0.20) a mile
http://codes.findlaw.com/ca/government-code/gov-sect-68093.html