my understanding is:
it is a check against my income with a random roll of 1-1,000,000 for a new CMC.
it is a check against my income with a random roll of 1-4,000,000 for an expansion to an existing CMC.
So the expansion, which happens fairly often is less likely. But I think that the CMC showing up is around 1% since the chance the check is made in Gl493.1 is well low as it has to fail in every other system as most of them have no sites to be selected...and this explains why I have only seen them in Sol. I need to get the systems with sites to have higher populations or so I think at the moment. I had not realized once the check succeeds that the process stopped even if no site was in the system.