Barack Obama last month spoke in New York to clients of Northern Trust Corp. for about $400,000, a person familiar with his appearance said. Last week, he reminisced about the White House for Carlyle Group LP, one of the world’s biggest private equity firms, according to two people who were there. Next week, he’ll give a keynote speech at investment bank Cantor Fitzgerald LP’s health-care conference. Obama is coming to Wall Street less than a year after leaving the White House, following a path that’s well trod and well paid.