An invasiion was not necessary thus your (highly dubious) million dollar figure could have been reduced to zero AND the bombs could have been avoided.. We could have agreed to a conditional surrender at the outset (as Truman finally did) to let the Japanese keep the emperor. Sorry to burst your bubble but it was not necessary to INTENTIONALLY kill thousands of babies, Christians (Nagasaki was the heart of Japanese Christianity), and little old ladies to promote terror.
The Japanese were, in fact, sending out feelers for a negotiated surrender through their ambassador in Japan. Stalin, unbeknowst to the Japanese, was not forwarding these feelers on to the Western Allies. Since the U.S. was reading Japanese codes, Truman was aware of them nonetheless.
The Japanese offer was cession of hostilities, no occupation of the Japanese home islands and continuation of the Japanese occupation of Korea, Formosa and Manchuria. To say that such a position was a non-starter would be an understatement.
Hiroshima was the headquarters of the Japanese Second Army (the aim point, btw), which would have taken the lead in defense of Kyushu, making it a legitimate target.
Contrafactuals can be fun, but in August 1945 there were no good options and Hiroshima seems to me to have been the least worse. No one can ever know how a "what-if" would have turned out and seconding guessing is America's favorite spectator sport. Truman did not have the luxury of being a spectator or engaging in contrafactuals. You are entitled to your opinion, but it seems to me that it is based not only on lack of evidence, but held in the face of mountains of countervailing facts.