As yet no evidence is available about the cost-effectiveness of positive psychological interventions. When offered via the Internet these interventions may be particularly cost-effective because they are highly scalable and do not rely on scant resources such as therapists’ time. Alongside a randomized controlled trial of an online positive psychological intervention a health-economic evaluation was conducted. Mild to moderately depressed adults seeking self-help and recruited in the general population were randomly assigned to the intervention group (n = 143) and a waitlisted usual care group (n = 141). Improved clinical outcomes were achieved in the intervention group at least for depression) at higher costs. When outliers (the top 2.5% n = 5 in intervention group n = 2 in control group) were removed cost-effectiveness was increased considerably. For positive psychology economic evaluations may be a means to nudge policy decision-makers towards placing positive psychological interventions on the health agenda.