Methods: Patients (n = 244) were randomized to receive either care-as-usual or a lifestyle intervention in which mental health nurses coached patients in changing their lifestyle by using a web tool. Costs and quality of life were assessed at baseline and at 6 and 12 months. Incremental costs per centimeter waist circumference (WC) lost and per Quality-Adjusted Life Year (QALY) gained were assessed. Budget impact was estimated based on three intervention-uptake scenarios using a societal and a third-party payer perspective. Results: Costs and reduction in WC were higher in the intervention (n = 114) than in the control (n = 94) group after 12 months, although not statistically significant, resulting in €1,370 per cm WC lost. QALYs did not differ between the groups, resulting in a low probability of the intervention being cost-effective in cost/QALY gained. The budget impact analysis showed that for a reasonable participation of 43%, total costs were around €81 million over 5 years, or on average €16 million annually (societal perspective). Conclusions: The intervention is not cost-effective at 12 months and the budget impact over 5 years is substantial. Possibly, 12 months was too short to implement the intervention, improve cardiometabolic health, and reduce care costs. Therefore, the incentive for this intervention cannot be found in short-term financial advantages. However, there may be benefits associated with lifestyle interventions in the long term that remain unclear.