WASHINGTON (Reuters) -The U.S. Department of Defense is nearing an agreement with Lockheed Martin Corp (NYSE:LMT) for around 375 F-35 fighter jets over three years, three sources said on Monday, amid expectations of a price increase for the most common version of the jet due to lower quantities and inflation. The first F-35A cost $221 million when it came off the production line in 2007. Since then, production quantities and know-how have increased, helping the price of the stealthy fifth-generation fighter fall to $79 million as it gained buyers. The handshake agreement would come as the aviation industry gathered for...