This working paper by UNICEF examines the economic cost of failing to address child marriage in Nepal.
It finds that, from a labour market perspective, the cost of child marriage amounts to 3.87 percent of the Gross Domestic Product (GDP) but is likely far higher, once the costs to girls’ health and psychological wellbeing are taken into account.
This is not to say that Nepal would gain this percentage increase immediately but that the loss from earnings over a girl’s productive years would be a substantial addition to the country’s GDP.
In particular, the paper looks at child marriage as an impediment to better educational outcomes, which in turns limits the country's productivity.