NBC and cable networks led by USA “were trading at a 20% discount to our competition” in the cost-per-viewer of ad sales before the recent upfront market, NBCU chief Steve Burke told analysts this morning. “We’re now at about a 10% discount.” Comcast’s entertainment arm says it bucked a trend in the upfront — seen as generally down 5% vs last year — as it benefits from the growing popularity of its shows, and a decision to sell broadcast and cable ad inventory together.