Pankaj Ghemawat argues that, as a rule of thumb, US markets have only been about 10% international vs. 90% domestic. Unfortunately, his article no longer includes this graph showing some of his data:
Exporters are in the minority. In 1992, only 21% of U.S. manufacturing plants exported anything. And the minority of manufacturers who do export still sell most of their output domestically: around 2/3 of exporters sell less than 10% of their output abroad.