The capex comprises $200 million on a mine and beneficiation plant, and the balance on a Wyndham Port refinery facility.
The latter provides the opportunity of hydroelectric power being incorporated.
The pre-tax IRR was put at 29%.
A pre-feasibility study is due to be completed next year.
As currently envisaged the mine would produce a rare earth mineral concentrate and a phosphate mineral concentrate.
"Further opportunity exists to produce a combined phosphate and rare earth mineral concentrate at site - allowing for reduced site infrastructure - with separation and processing at port," RareX said.
"This should result in a substantially larger mining operation."
Other upside is in the resource which is seen by the company as having "substantial growth potential".
RareX said it had cash and investments totalling $10.3 million at the end of June.