Ann: Quarterly Activities/Appendix 5B Cash Flow Report, page-15

This is a somewhat rudimentary method to determine that $100M p.a free cash when in full production is a fanciful idea.

As you state, you have no idea where C1 costs will end up after steady-state is reached but it is fair to say that it will not be a simple extrapolation of costs from the maiden 'full' quarter during initial ramp-up and commissioning.

I would suggest Orion may have done a little bit of modeling beyond the first quarter of commissioning to ascertain whether risking their US$110m made economic sense.

And just to clarify:
C1 Cash costs already account for General & Administrative expenses ('corp cost'). I suggest those interested read the Additional Information on page 10 of SFX announcement.
Free cash flow is the money a company has left over after paying its operating expenses and capital expenditures. It is with this free cash flow that companies then repay debt (thus increasing net assets), and subsequently pay dividends to shareholders and fund growth opportunities.