After the MDA, page-11

The Feasibility Study showed a US$1B NPV of the Project at commencement of operations and before tax using a discount factor of 8%.
From recollection GBE has stated tax over the life of the Project is expected to be about US225M, the Government will be obtaining a 10% interest in the Project and, of course, there will be some time passing before the Project commences in what is now a staged process. Investors would probably use a higher discount factor although at the time of commencement some of the main risks (in particular, Malawian Government delays) will have been minimised.
So pluck a figure out of the air for what the SP will be once the MDA is signed and also how the SP might increase as various milestones such as financing (capital requirements were relatively speaking not that high and with a staged process will initially be even less) and sales arrangements are released to the Market.