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.
No, I understood your point. The problem was, your comment about the unreliability of the statistics presented by Tony Snow -- it was a good one -- also applies in the other direction: we can no more trust the statistics cited to show that illegal immigration is on balance "bad".
As to whether illegals "should not be here to begin with," I think it's clear that many Americans disagree, to the point that they're paying people to cross the borders. There is clearly a market for illegal immigrant labor.
The sticking point for you is the "illegal" part. Suppose, however, that a way was created for this large group of people to be here legally -- say, some sort of guest worker program that allows us to track and tax those who currently are not tracked and taxed. Would your argument about "they shouldn't be here" be addressed, or would you find some other reason to oppose their being here?