Although the mathematical ability of Taiwanese students is ranked high according to the Programme for International Student Assessment (PISA), Taiwan faces the problem of a gap in test scores between rural students and their non-rural counterparts, with the latter group scoring higher on average. Inadequate funding is a possible reason for this discrepancy. Therefore, this paper uses the cost function approach to compare adequate funding requirements between rural and non-rural schools. The adequate funding estimates are calculated based on the different levels of mathematics scores in the Basic Competence Test (basic, average, and proficient). The results showed that rural schools require more per student funding than do non-rural schools, irrespective of the use of score levels as references. A greater portion of rural schools were shown to operate under inadequate per student funding compared with non-rural schools. This paper provides further evidence of the urgency of increased investment in rural schools.