Ann: Half Yearly Report and Accounts, page-15

Up to US$120/tonne according to this article dated October 2017:

Unlike high-value minerals such as gold, manganese has low commercial value with the average price over the past ten years sitting at around Rp.1000 per kilogram (US$0.08), although it can fluctuate between Rp.500–1500 (US$0.04–0.12) per kilogram.

Manganese prices have risen since October, but from what I can find online, they're not quite at 5-year highs. It should be noted the article is also referencing artisanal mining, so you'd expect whoever is paying that to the local farmers is putting a markup on top before onselling in bulk. Unless GMC are dealing direct, but that seems a very inefficient way to operate a smelter.

Power costs @ US$0.11/kWh are US$360/tonne according to the Kupang Project metrics, not US$440/tonne. I expect GMC will be able to secure power supply for considerably less judging by the latest PLN tariff prices released earlier this month (google translate required).

The most expensive tariff is now US$0.10/kWh, which is also for households. Industrial tariffs range between $0.034 (subsidised) and $0.078. Those categories only go up to 200 kVA though, when GMC have secured up to 20 MVA. It's difficult to know what price they will be paying for power.