If everyone had access to healthcare the net benefit of treating the mental illness and other disabilities holding them back would easily cover the cost of the healthcare itself.
I bet the benefit of free school lunches would also pay for itself. And there's been studies that show that funding early childhood education has a huge ROI.
As someone in the final stages of a masters degree in healthcare management and economics:
Almost. It doesn't entirely cover the costs (at least from the data we have available worldwide, which is somewhat insufficient) but a focus on mental health(which always includes workers rights, women's rights and a few more social issues that create long term health problems on a massive scale) and prophylaxis in general is FAR cheaper than what most industrial nations currently do.
We do have a few issues that are not addressed in these concepts (e.g. end of life care and costs associated with that, new types of personalised medication, accessibility in rural areas,etc.) that still make a healthcare system like that something society has to pay for...But it does improve things massively, especially the quality of life of people that are not the actual patients.