Gilead Sciences Inc., whose $1,000-a-pill hepatitis C treatment is one of the world’s most expensive drugs, is avoiding billions of dollars in taxes by booking profit overseas. The data released in a securities filing this week suggest that Gilead is shifting valuable intellectual property to low-tax countries and paying about 5 percent in taxes on its foreign income, said independent tax consultant Robert Willens. Gilead’s disclosure is the latest example of tax planning by U.S.