The results of a huge, seven-year study in rural South Africa are in, and god damn: by giving antiretroviral drugs daily to 100,000 people in Kwazulu-Natal, researchers were able to raise the entire region's life expectancy by more than a decade. From the LA Times:
In 2003, the year before the drugs were available, 29% of all residents were infected with HIV and half of all deaths there were caused by AIDS. Life expectancy in the region was just over 49 years.
By 2011, life expectancy had grown to 60 1/2 years - "the most rapid life expectancy gains observed in the history of public health," said study senior author Till Barnighausen, a global health professor at the Harvard School of Public Health.
The entire study cost $10.8 million, and was deemed "highly cost-effective" based on the benefits it showed. A separate study conducted in the same region at the same time showed that "Healthy individuals in [areas with antiretroviral drug interventions, like Kwazulu-Natal] were 38% less likely to contract HIV than people in areas where ART drugs were not widely available."
We could conduct this study 40 times over for the price of a single US Navy Littoral Combat Ship, which the Navy doesn't even need.