What is the test for awarding expert witness fees as a section 998 discretionary item of costs?

We review the trial court's award of expert witness fees as a section 998 discretionary item of costs using an abuse of discretion standard. (Evers v. Cornelson (1984) 163 Cal.App.3d 310, 314, 209 Cal.Rptr. 497.) Appellants first contend that the trial court abused its discretion in awarding respondent discretionary costs because the section 998 offer to compromise was not a reasonable offer, but rather a token, tactical one made only to preserve the right to later claim these disputed costs.