In May, Inmet increased the cost estimate for Cobre Panama by 43 percent, according to The Globe and Mail. Inmet's share of the $6.2 billion development cost will be $4.8 billion. Korea Panama Mining, which owns 20 percent of the mine, will put down the rest. Inmet had nearly fully funded the project at the end of September, with the exception of $371 million, and in December, it raised $500 million by selling senior notes, part of which will be used to fund Cobre Panama.

Inmet said it “recognizes the industry trend of escalating costs as a significant risk” and to limit cost escalation it has so far contracted out $4.1 billion of the total $6.2 billion costs. But the costs may be on the high side, and shareholders may want to consider what another owner could do to limit them.

Inmet's cost estimate is at about $23,000 per tonne of copper produced, according to Nomura analysts cited by Reuters. That's well above the $18,500 sector average, and almost four times the $6,000 per tonne that First Quantum expects to spend to develop its Sentinel copper mine in Zambia. First Quantum did not respond to requests for a detailed cost overview for Sentinel.

For First Quantum, acquiring Inmet is an opportunity to establish itself as one of the five biggest copper producers in the world amid a continued global supply shortage. It is also confident that it can build Cobre Panama more cheaply. “We tend to build [mines] at somewhere between a third and half the costs of anybody else,” President Clive Newall told The Globe and Mail.

Having already raised its bid twice, First Quantum might be amenable to raising it again. “Our clear preference remains to engage with Inmet, as we believe strongly in the compelling strategic and financial merit of the transaction,” Philip Pascall, CEO of First Quantum said in a statement. If Inmet continues to resist, and First Quantum or other bidders don't offer more, the company's shares may fall back to the C$40-something level they were trading at before the first bid.