Source: DESTINATION MOON: A History of the Lunar Orbiter Program, NASA TM-3487

[303] Even before Lunar Orbiter V flew, the Office of Space Science and Applications was entertaining the prospect of flying a sixth Orbiter mission. Boeing had nearly enough parts to assemble another spacecraft at an initial cost of about $13 million. A gamma-ray experiment also existed which scientists desired to fly on a sixth Orbiter. Its inclusion would raise the cost of the mission by about $3 million. However, the necessity to relocate personnel on the Lunar Orbiter team to other jobs presented a major problem blocking another mission.¹