Mo. Rev. Stat. § 506.369: Partial payment of court costs, when.

1. If the court receiving a motion pursuant to section 506.366 determines that an offender is unable to pay the full amount of court costs due with respect to a case, the court shall assess a partial payment of the twenty percent of the greater of the following:

(1) The average monthly deposits to the offender's account for the six-month period immediately preceding the filing of the complaint or notice of appeal requiring the payment of a fee; or

(2) The average monthly balance in the offender's account for the six-month period immediately preceding the filing of the complaint or notice of appeal requiring the payment of a fee.

2. If a trial court has entered an order pursuant to subsection 1 of this section upon the filing of plaintiff's petition, the determination of the court shall apply to the case until final judgment is entered by either the trial or an appellate court.