I would have thought that some studies would need to be carried...

I would have thought that some studies would need to be carried out and considered before an FID takes place in May. These are said to have been completed yet nothing has been presented to shareholders or have I missed it? MIN are undergoing some studies expected to be released to shareholders soon regarding costs for the plant they are proposing.
This is what PLS holders get told, from recent announcement;

"Proposed chemical conversion plant offers a comparable capital and operating cost base to recently established/constructed and operating lithium chemicals projects"

What is that supposed to mean?

Based on those comments, the most recent cost guidance that I know of is for the Tianqi plant (who already process lithium/have experience in this space). They have guided $700m for 48 ktpa, which is fairly in line with the $US400m 40ktpa from CG for Posco taking into account cheaper labour in South Korea and 8ktpa less plant size but costs have blown out towards or over $1B by some reports at Tianqi. We can also cite the cost blowouts at Nemaska as another example (to a lesser extent) of the struggles Posco/PLS are likely to face. Red flag no. 1

"Patented PosLX electrodialysis processing technology developed over more than eight years"

How many times have we seen these "groundbreaking processing techniques" fail? In the lithium space we have Nemaska once again, new tech, big blowouts and delays very likely and there are plenty of other examples in other commodities that prove this point. Joe Lowry touches on this also. Red Flag no. 2

But to answer your question, capex estimations are from CG at $US400m and
"CG have said they expect downstream participation to be value accreditive and increase group earnings by 12%."

Long story short, the odds of cost overruns are far more likely not, just by how much.
High risk/not so high return endeavour to increase group earnings by 12%.

In other news that bat stew was sensational.