How much is an endorsement from Oprah Winfrey worth on Broadway? So far, about a couple million.Winfrey, one of the producers of the new tuner “The Color Purple,” featured the production on her talkshow Friday, and sources familiar with “Purple” B.O. say that since the segment aired, single ticket sales have topped $1 million and group sales have come in between $750,000 and $1 million. That puts the advance for the $10 million show, set to open Dec. 1, at a respectable, if not astonishing, $6 million. And more sales could be on the way: Scott Mallalieu, prexy of Group Sales Box Office, said that on his org’s Web site, requests for group sales info about the show have more than quadrupled. The musical’s wraps had been building steadily over preview period. The surge, though, echoes the recent frenzy over “Jersey Boys,” whose enthusiastic reviews last week helped it rake in more than $1 million in single ticket sales over just a couple of days.