The aim was to calculate the total and the net costs per child included in a 3-year caries preventive program for preschool children and to make estimates of expected lowest and highest costs in a sensitivity analysis. The direct costs for prevention and dental care were applied retrospectively to a comprehensive oral health outreach project for preschool children conducted in a low-socioeconomic multi-cultural urban area. The outcome was compared with historical controls from the same area with conventional dental care. The cost per minute for the various dental professions was added to the cost of materials, rental facilities and equipment based on accounting data. The cost for fillings was extracted from a specified per diem list. Overhead costs were assumed to correspond to 50% of salaries and all costs were calculated as net present value per participating child in the program and expressed in Euro. The results revealed an estimated total cost of 310 Euro per included child (net present value) in the 3-year program. Half of the costs were attributed to the first year of the program and the costs of manpower constituted 45% of the total costs. When the total cost was reduced with the cost of conventional care and the revenue of avoided fillings, the net cost was estimated to 30 Euro. A sensitivity analysis displayed that a net gain could be possible with a maximal outcome of the program. In conclusion, the estimated net costs were displayed and available to those considering implementation of a similar population-based preventive program in areas where preschool children are at high caries risk.