Many people with dementia live in the community; thus, supporting informal caregivers is critical. The Connected Health intervention facilitates collection and sharing of patient data among informal caregivers and providers to identify emerging patient needs and support rapid decision-making. This study estimates the costs of care of dementia using time-driven activity based costing of an exemplar patient. Intervention costs and health utility values were derived from a feasibility study of the intervention. A Markov model produced estimates of the cost-effectiveness of the intervention under four scenarios: (1) a minimal effect of the intervention on disease progression; (2) moderate effects on disease progression, and minimal effects on quality of life (QOL) and cost; (3) minimal effects on disease progression and QOL, and a moderate effect on cost; (4) moderate effects on disease progression and cost, with minimal effects on QOL. Cost estimates of formal and informal care ranged from €3713 to €7614 per month. Intervention costs were €484 per month. Under scenarios 2, 3 and 4, the cost per quality-adjusted life year of the intervention falls below €45,000, the threshold below which the Health Information and Quality Authority in Ireland generally accepts interventions as cost-effective. The results suggest that the intervention would be cost-effective with limited reductions in rates of disease progression and cost of care, and with minimal improvements in quality of life. Future research should consider the specific experiences of intervention patients.