News: BHP BHP Group Announces Agreement To Pursue A Merger Of Petroleum Business With Woodside, page-31

In the most literal sense BHP will make $0 from the divestment, the merger will happen at the shareholder level. Woodside are not paying cash for the assets, they are issuing stock to existing BHP shareholders in consideration for the assets.

For examples sake as BHP shareholder you would have $50 of value pre the merger via 1 BHP share. After the merger you would have $30 of BHP shares and $20 in Woodside. In a perfectly efficient market you are no better, and no worse. See the Woolworths and Endeavour divestment as an example.

The exact split and value is still to be determined as this is only on the asset level. I imagine BHP would want to shift some of the significant remediation liabilities to Woodside and as such the NTA contributions to the merger will not be 48/52.