The SWAP IT program aims to improve the nutritional quality of school lunchboxes via a multicomponent m-health intervention, involving: weekly support messages to parents; physical resources; school nutrition guidelines and lunchbox lessons. SWAP IT has been reported to be effective. This study aims to determine the cost and cost effectiveness of the SWAP IT m-health intervention. The retrospective trial-based economic evaluation was conducted in 12 Catholic primary schools in New South Wales, Australia. Schools were randomised to intervention or usual care. The costs (AUD, 2019) were evaluated from societal perspectives. The direct cost to uptake the intervention and the incremental cost-effectiveness ratios (ICER) were calculated. ICERS were calculated for two outcomes: reduction in total kJ and reduction in discretionary kJ from the lunchbox. The total cost was calculated to be AUD 55, 467. The mean incremental cost per student to receive the intervention was calculated to be AUD 31/student. The cost per reduction in total lunchbox energy was AUD 0.54. The ICER for the reduction in energy from discretionary foods in the lunchbox was AUD 0.24. These findings suggest that this m-health intervention has potential to be cost effective in reducing the kilojoules from discretionary foods packed in school lunchboxes.