Based on the invasions of Iwo Jima and Okinawa, the US forces were predicting US casualties of the order of one million for an invasion of the main Japanese islands. This was one factor in the decision to use the bomb. However, at the time, Japan was totally prostrate. Their air defence was non-existent, US submarines were preventing most supplies of anything getting through and US fighter-bombers roved and wrecked at will on the main islands. And the fire raid on Tokyo by B-29s killed more people than died in Hiroshima (discounting later radiation deaths). Perhaps a reasonable offer would have saved a lot of lives - nobody knows because it was never made. And, as Michael says, the bomb was a demonstration to Stalin not to push too hard.

I still think that turning a small Japanese island into glass would have been a much better way to do business.