Nine Entertainment has begun negotiations to sell its controlling stake in Domain to US real estate conglomerate CoStar, with both sides debating a price at which Nine would be comfortable to sell its key growth asset.

CoStar launched a sharemarket raid on Domain last month, snapping up a 16.9 per cent stake, and then made a cash offer for the remainder of Australia’s second-biggest property listings platform, priced at $4.20 a share.