What is the burden of proving that a judgment is more favorable than the offer?

" 'Where, as here, the offeror obtains a judgment more favorable than its offer, the judgment constitutes prima facie evidence showing the offer was reasonable and the offeror is eligible for costs as specified in section 998. The burden is therefore properly on plaintiff, as offeree, to prove otherwise.' [Citation.]" (Carver v. Chevron U.S.A., Inc. (2002) 97 Cal.App.4th 132, 152 [italics added].) Thus, "[i]f the items appearing in a cost bill appear to be proper charges, the burden is on the party seeking to tax costs to show that they were not reasonable or necessary. On the other hand, if the items are properly objected to, they are put in issue and the burden of proof is on the party claiming them as costs. [Citations.] Whether a cost item was reasonably necessary to the litigation presents a question of fact for the trial court and its decision is reviewed for abuse of discretion. [Citation.] However, because the right to costs is governed strictly by statute [citation] a court has no discretion to award costs not statutorily authorized. [Citations.]" (Ladas v. California State Auto. Assn. (1993) 19 Cal.App.4th 761, 774-775 (Ladas). See also Carver v. Chevron U.S.A., Inc., supra, 97 Cal.App.4th at p. 142 ["de novo review of . . . a trial court [cost] order is warranted where the determination of whether the criteria for an award of attorney fees and costs in [a particular] context have been satisfied amounts to statutory construction and a question of law"].)