US real estate giant CoStar has agreed to buy property sales portal Domain in a $3 billion deal that will create a more powerful, cashed-up competitor to News Corp’s REA Group, and deliver a windfall to Nine Entertainment shareholders.

After almost six weeks of due diligence, CoStar has agreed to pay $4.43 a share for Domain. This is a 60 per cent premium to the price Domain was trading at before CoStar first expressed interest in the business in February.