Autism spectrum disorders are associated with a substantial economic burden; although little is known about the relationship between state and family out-of-pocket expenditure. The objective of this study is to estimate the societal cost of childhood autism spectrum disorders and explain the variation in costs between state and family out-of-pocket expenditure. A bottom-up prevalence based cost-of-illness methodology was implemented using data from a combination of multiple convenience samples in Ireland of 195 parents of 222 children aged between 2 and 18 years of age with a clinically diagnosed autism spectrum disorder collected in 2014/2015. The findings show the average annual cost per child for families amounted to euro28,464.89 related to private autism spectrum disorder services, lost income and informal care. By comparison, annual state expenditure per child on autism spectrum disorder-related health, social and educational resources was euro14,192. Regression analyses indicate that autism spectrum disorder severity is significantly associated with higher out of pocket expenditures but not state health expenditures. The results suggest that parents are central to meeting the needs of young people with autism spectrum disorders in Ireland. Policy implications of these findings suggest that significant investment and commitment is needed to address the needs of individuals living with autism spectrum disorders and their families.