I think the memo actaully compared cost per hour which was more than the 707. But as the 10 was considerably faster, the cost of flying it across the Atlantic was about the same. And as mentioned, it was more popular with the passengers. Roomier, smoother and quieter. The 707 dutch rolled it's way across the Atlantic due to it not having a yaw damper making the passengers in the last few rows very queasy. BOAC filled the 10s, it was the 707s which had empty seats due to it being less popular.