That's just on the American/Allies side. I've read estimates that a land invasion would have caused upwards of 5 million Japanese civilian casualties.
In advance of a planned invasion, the War Department placed an order for 500,000 Purple Hearts. After nearly 70 years, that inventory has yet to be exhausted.
My late father was on a yard mine sweeper headed toward Tokyo in August 1945, when, for unexplained reasons, his group was ordered back to Okinawa. He did not expect to survive the pending invasion of the Japanese mainland.
He told me once that his ship shot down a kamikaze while they were stationed off Okinawa. The plane hit the water, broke into pieces, and the cockpit section wound up on deck. Fortunately, no one on board was hurt. When they put out the fire, they looked at the body of the pilot, who they figured was no more than sixteen, and wondered what kind of people would send their kids out to kill themselves in this way....
It also saved Japan from being divided into Soviet and American zones, with most likely a civil war resulting, just like Korea.