July 30 (Reuters) - Insurance broker Aon Plc (AON.N) said on Friday it could have to expend up to $400 million in additional costs tied to the termination of its mega merger deal with Willis Towers Watson , over and above the $1 billion it was required to pay. The extra costs, which would range between $350 million and $400 million, could be incurred in the third quarter, Aon Chief Financial Officer Christa Davies said on a post-earnings call with analysts. Earlier this week, the companies said they had mutually decided to pull the plug on their $30 billion merger that would have created the world"s largest insurance broker, due to regulatory objections. read more The termination of the deal was hailed as a victory for the Biden administration. Last month, the U.S. Department of Justice sued to block the merger. "Despite regulatory momentum around the world ... we reached an impasse with the U.S. Department of Justice," Aon Chief Executive Officer Greg Case had said. The U.S. regulators" stand was in contrast to that of their European counterparts who had approved the deal on condition the companies sell certain assets. read more London-based Aon on Friday reported a 5% drop in second-quarter net income attributable to shareholders.