Western Australia's opposition says its ambitious Metronet rail plan would cost about $3.8 billion, about half the amount estimated by the state government.

WA Transport Minister Troy Buswell has urged the state's Labor Party for weeks to provide costings for the plan, which would bring rail lines to some of the city's most isolated suburbs including Ellenbrook on the north-eastern fringes.

A map of the promised Metronet system that will include extensions to the rail network and new circle lines.

Mr Buswell says he expects the Metronet plan will cost $6.4 billion.

But Opposition Leader Mark McGowan said Mr Buswell "makes things up" and called on the scandal-plagued minister to reveal costings for the government's alternative light rail plan.