Here’s one affordable way to beat back a casual Japanese invasion:
USA starts with a sub off the coast of Hawaii, which you can submerge if you want during Pearl Harbor, since Japan has no destroyers in range on Japan’s first turn. Build one more submarine for 6 IPCs, and park your 2 subs off the coast of San Francisco. Japan can’t sink your subs with carrier aircraft unless they bring a destroyer with them. Meanwhile, you can build a bomber each turn in the Eastern US as part of your Kill Germany First strategy. If Japan ignores you, the bomber can go east to harass Berlin. If Japan brings a small transport fleet toward San Francisco, you can hit it and kill it with 2 subs and a bomber.
Of course, if Japan brings its entire fleet toward San Francisco, you’ll need bigger defenses – but you’ll see that coming and have time to stop it, and if Japan swings its whole fleet east, then that’s so many more turns that Siberia and India are safe.