Excerpt from Honor Among Thieves:
"The reason for numbers being limited on this occasion will become all too obvious to you. The issue we are about to discuss is so sensitive that the fewer people who are aware of it, the better. To suggest in this instance that silence is golden would be to underestimate the value of gold."
Here what does Jeffery means when he says "underestimate the value of gold"? Does it means gold is really precious and at this point silence is something that they couldn't afford and if they do that it will underestimate the value of gold ?