Ann: Altura Completes October Shipments of Lithium Concentrate, page-144

@Rupfel what I have seen is somewhat dated, from Tianqis 2018 prospectus, freely available for download in public domain.
There they show Greenbushes life of mine total operating cash costs (mining + processing + g&a site operating costs + transport + royalties) as A$275/t c.u$194/t.
This is quite conceivable when you consider the head grade at greenbushes is 2.0% on average, twice that of AJM and they have freak pegmatite veins with 3-4.5%. While the relationship between head grade and output of sc6.0 is not exactly 1 to 1, for simplicity I would assume they get twice the output for the same volume of ore input into the plant.
The other advantages are location related, they are c45min drive to a very large town and port. Which means they don't need a fifo roster for employees. From what I've heard, people are often willing to take a material pay cut for the better lifestyle, family time etc greenbushes offers vs remote areas such as the pilbara. The other benefit is power and water supply. Greenbushes is within the swis power network, drawing directly from the grid as opposed to having to rely on diesel.
With that said, I think you are right about sqm and oversupppying the market. Orecobre seems to be taking a similar approach. It's a tough market out there.
What you would have seen in the press on greenbushes holding back expansions relates to planned chemical grade plants (cgp) 3 and 4. Cgp1 + 2 + 3 + 4 would have a combined output capacity of 2.4mtpa of sc6 with reserves supporting a mine life of 10-15years at the fully expanded production runrate. Incredible.
Hope that helps