China is investigating some of the nation’s top investment banks over bond underwriting fees that hit below $100, as regulators expanded a crackdown on a long-standing price war amid broader efforts by President Xi Jinping to curb cutthroat competition. The National Association of Financial Market Institutional Investors last week launched a probe into the six underwriters of a 35 billion yuan ($4.9 billion) debt sale by China Guangfa Bank Co. after their fee bids “drew public attention.”