Ann: Quarterly Activities Report and Appendix 5B, page-107

We don't know how much it will cost to build. The point of the trade off analysis was to figure out how to get the most bang for our buck, as @vmp has pointed out.

If you can double capacity for spending something like 20% more (at a guess), that's pretty compelling. Definitely improves the NPV.

As for taking longer to build, we haven't got any indication that it would. Again, the idea was to look at getting off-the-shelf parts rather than having stuff bespoke made. Pretty rare for bespoke production to be quicker than off-the-shelf.

If you mean it takes longer to build because they've had to push the feasibility study back, then yes, that has had a negative impact on NPV. But that impact is just another year of discounting back cashflow.

So if you were working on an NPV of $X for 2500tpa, a 5000tpa plant will have an NPV of $2X.

Unless your discount rate for the extra year is greater than 50%, when 10-12% would be standard, then $2X/(1 + discount rate) is always going to be greater than X.