Thanks for missing the point completely -- the point was not addressing the number of illegals in prison, but the number of illegals outside prison, that were stated to be "unknown." If you don't know how many total illegals there are, you cannot expect to make any serious statistical inferences about those in prison, as a proportion of the "illegal" population.
You missed the point, you seem to be concerned with what percentage of illegals are in prison, most of us don't care what percentage of illegals are in prison of the total illegal population. The fact is illegals should not be here to begin with, so we don't want to spend any money on their incarceration. Therefor any money spent on illegals that are in prison is an amount we should not be spending. It would not matter to me if only a very small number of illegals by comparing to total illegal population were in prison- how ever many it is is too many.