It was speculated at the time that there would have been close to 1,000,000 American combat dead, and at least 10,000,000 Japanese combat dead.
Also, let us not forget the Japanese started the damned war and all we did was finish it, so by dropping the bombs, we saved 1,000,000 American lives and at least 9,500,000 Japanese lives.
By forcing an unconditional surrender, we were able to rid the Japanese of their militarists which were responsible for multiple atrocities, such as the Rape of Nanking, the activities of Unit 731 (the Japanese unit which used local civilians and POWs for chemical and biological weapons experiments and the Bataan Death March. We institute a Western style form of government and Japan has prospered since the end of the war.
I am personally sickened by the number of apologists we have regarding how the war was forced to conclusion. Had Truman not ordered the use of the A-bombs, both me and my wife would not be here, because both our fathers would have undoubtedly been killed during the invasion and it's subsequent fighting.
My dad served in Europe and his unit was on the way to the Far East when the bomb was dropped.