Additional description of the costing methods.

<div><p>Suubi4Her is a combination intervention that integrates a savings-led family-based economic empowerment intervention through youth development accounts with a family strengthening intervention delivered via multiple family groups. It aims to improve mental health and reduce HIV risk behaviors among school-going adolescent girls in Uganda. This micro-costing study was conducted as part of a three-armed randomized control trial between 2017–2022, involving 1,260 participants aged 14–17 years across 47 secondary schools. Adopting a provider perspective, we prospectively identified, measured, and computed the costs associated with all program activities. These costs were then aggregated and divided by the actual number of adolescent girls in each study arm to conservatively obtain the per-adolescent costs for each arm. The per-adolescent costs of economic empowerment intervention alone and in combination with the family strengthening intervention were US$476 and US$812, respectively. Personnel costs were the key cost driver due to the intensive supervision of intervention delivery and quality assurance efforts. This study is the first to estimate the economic costs of an evidence-based combination intervention targeting the multifaceted risk factors underlying HIV risk among adolescent girls in a low-resource setting. The per-adolescent cost of US$812 for the Suubi4Her intervention falls within the cost range reported for other family-based interventions (US$500-US$900); however, published comparisons are limited. Accurate and reliable cost estimates are key to assessing the feasibility, affordability, and economic value of interventions. There is a pressing need for more costing studies on evidence-based combination interventions, especially in low-resource settings (Trial registration: Clinical Trials NCT03307226; IRB approvals: Washington University in St. Louis (IRB #201703102), the Uganda Virus Research Institute (GC/127/17/07/619), and the Uganda National Council of Science and Technology (SS4406).</p></div>