The fast-track merger talks between United Airlines and Continental Airlines have become bogged down in disagreements over share price, sources say. Share price is crucial because any merger would be completed through a stock swap. United shares have jumped 20% in price since rumors of a merger with US Airways first emerged on April 7. But Continental emerged as a major contender only a week ago, and its stock has risen about 5% in the shorter time frame. To protect shareholder value, sources say Continental wants to use the 30-day average stock price prior to April 7, while United wants to price the deal at current levels just before signing.