The average cost of making a major movie topped $20 million for the first time last year, Jack Valenti, president of the Motion Picture Assn. of America, said Tuesday. The cost of prints and advertising, Valenti said, pushed the 1987 average cost of producing and releasing a film to more than $30 million--not including corporate overhead, salaries, development or distribution expenses. Valenti told an industry audience attending the ShoWest convention in Las Vegas that production costs are rapidly outstripping revenues in Hollywood. (For comparison, the average major film in 1980 cost $9 million, and $17 million in 1986.) Twenty films released in 1987--out of a total of more than 400 released--made more than $20 million in rental revenue, Valenti said.