Understanding minimums in the calculation of Human Development Index In the calculation of HDI, three indices are involved. One of them is the GNI index. In calculating this index, there's some scaling done. Quoting from the UNDP Technical notes 


  The low minimum value for gross national income (GNI) per capita, $100, is justified by the considerable amount of unmeasured subsistence and nonmarket production in economies close to the minimum, which is not captured in the official data.


I don't understand what this justification means. Isn't \$100 too low? The index goes on to put an upper cap at $75000 which makes sense - because beyond that income there's not much impact on the standard of living.

The calculation of this index is fairly straightforward:

$Dimension\ index = \frac{actual\ value – minimum\ value}{maximum\ value – minimum\ value}$
This means that if a country would have most of its population living out of subsistence farming, which in practice could be registered as zero income, this would lead to an estimated/assumed GNIpc equivalent to \$100 per year per capita, which no country would ever go below. 
Having this value makes sense when reading the starting notes of that section:


  Minimum and maximum values (goalposts) are set in order to 
  transform the indicators expressed in different units into indices on a scale of 0 to 1. These goalposts act as the “natural zeros” 
  and “aspirational targets” (...)


Said in other words, having this minimum means that \$100 would be the GNI per capita that would provide the lowest or minimum standard of living possible.

Answering to the "isn't \$100 too low": 


  For income, it is set at \$100 per capita GNI, which is lower than the lowest value attained by any country in recent history (Zimbabwe in 2008). Should any country’s per capita GNI fall close to or below $100, the minimum will be changed accordingly.


I can recommend to read the source from UNDP, gives interesting inputs on the other dimensions too. 

There are mathematical reasons to chose a low minimum

It is also convenient to choose minimums that make the dimensions non-zero, or not too close to zero even for countries which would score close to the minimum. Since the HDI is all about the combination of several aspects, and the formula is:

$HDI = (I_{health} \cdot I_{education} \cdot I_{income})^\frac13$

If any country would score a zero in any dimension this would dominate the whole HDI (make it zero), which defeats the purpose of the index. The index aims at acknowledging average human development balancing the three aspects.