If I remember correctly at most 2 million copies of SQ42 have been sold so far, but when it is released I'll be surprised if it doesn't sell well over 10 million copies the first year.
As for costs to maintain the SC as well as to continue to add content and more ships, I'm anxious to find out how that'll be funded. I know CR has stated it'll be complete at release, I feel that is naive. If it is to be realistic as CR has stated then the manufacturers are would be developing new weapons, components, armor, and ships, real ones never stop developing things to sell to stay relevant, plus new systems will have to be added periodically in order for exploration to remain engaging.