If a witness won’t come to trial and testify for you voluntarily, then yes, you need to pay your witnesses.
Expert witnesses set their own expert witness testimony fees. Some experts may charge under $100 per hour, but some experts charge hundreds, even thousands of dollars per hour.
Plain old fact witnesses who won’t show up at court as a favor to you must also be paid, but they are only required by law to be paid $18.50 on the first day of trial, and if they are needed to testify more than one day, the law requires that they be paid $49 per day. If the witness has to travel more than 50 miles to get to court, you have to pay the witness a mileage fee too. The details of witness fees are provided below in Utah Code § 78B-1-119
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78B-1-119. Jurors and witnesses — Fees and mileage.
(1) Every juror and witness legally required or in good faith requested to attend a trial court of record or not of record or a grand jury is entitled to:
(a) $18.50 for the first day of attendance and $49 per day for each subsequent day of attendance; and
(b) if traveling more than 50 miles, $1 for each four miles in excess of 50 miles actually and necessarily traveled in going only, regardless of county lines.
(2) Persons in the custody of a penal institution upon conviction of a criminal offense are not entitled to a witness fee.
(3) A witness attending from outside the state in a civil case is allowed mileage at the rate of 25 cents per mile and is taxed for the distance actually and necessarily traveled inside the state in going only.
(4) If the witness is attending from outside the state in a criminal case, the state shall reimburse the witness under Section 77-21-3.
(5) A prosecution witness or a witness subpoenaed by an indigent defendant attending from outside the county but within the state may receive reimbursement for necessary lodging and meal expenses under rule of the Judicial Council.
(6) A witness subpoenaed to testify in court proceedings in a civil action shall receive reimbursement for necessary and reasonable parking expenses from the attorney issuing the subpoena under rule of the Judicial Council or Supreme Court.