Where did the creators of the Six Million Dollar Man come up with that dollar amount?

From Wikipedia:

The show is based on the novel Cyborg by Martin Caidin, and during pre-production, that was the proposed title of the series. ...

Austin is severely injured in the crash and is “rebuilt” in a title-giving operation that costs at least six million dollars. His right arm, both legs and the left eye are replaced by "bionic" implants that enhance his strength, speed and vision far above human norms: he can run at speeds of 60 miles per hour (100 km/h), and his eye has a 20:1 zoom lens and infrared capabilities while his limbs all have the equivalent power of a bulldozer. He uses his enhanced abilities to work for the OSI (Office of Scientific Intelligence) as a secret agent (and as a guinea pig for bionics).

In the book, the $6m covers the cost of the operation and also maintenance and associated costs to keep Austin operational for five to nine years. From chapter 5:

"My reaction is not the issue. You are talking about years of work, Mr. Goldman. What is involved here concerns the active participation of much of my laboratory, both the public facility, and, of course, our secret center. You are talking about dozens of skilled doctors, engineers, technicians, the use of at least one and perhaps two computers critically needed in other areas. We can promise you no real measure of success, and—"

"May I break in, Dr. Killian?" Goldman asked quietly. "Perhaps I can bring things to a head. What do you estimate would be the financial requirement for this program? More specifically, as it involves Colonel Austin."

"Two million the first year, perhaps. After that it is difficult to tell. I would say from a half million to twice that much every year for some time to come."

"Doctor, tomorrow by this time there will be placed within your fiscal control—nonreturnable so long as this project is under way—six million dollars."