North’s formal response to rival bidder, Rio Tinto, lodged late on Friday, revealed a deal that enables Anglo to claim up to $37 million in costs from North if its bid is not accepted.

South African giant Anglo entered the fray with a $4.20 per share counter-bid to Rio Tinto’s $3.80/share, or $2.8 billion, offer on Friday. The bid has the backing of North’s board and its Japanese joint venture partners.

Rio Tinto said that it would study North’s detailed response to its hostile bid before deciding its next move. It expected to make a decision this week.

An independent report from Grant Samuel & Associates released at the weekend valued North at between $3.21 billion and $3.75 billion, or $4.34 to $5.09 per share, well above either takeover offer.

North, meanwhile, issued an independent valuation which said the company could be worth more than $5 to Rio because of large synergies and resulting savings available to the British-Australian giant.

North’s shares moved 11c higher to $4.45 in early trading on Monday, and Rio was 2c higher to $25.90.