What makes human beings valuable?
Value has rules. Outside these rules, a valuable thing cannot have value. Here we must consider what we as humanity should value. This is what makes a person's value. Culturally, the values are talents, abilities, capabilities, good characters, etc. Education, wealth and beauty are most sought after. When men see a beautiful girl walking along the street, they would keep their eyes on her following. Strength, health, mannerism... are personal values but they need to conform with a culture.
So value is within the rules. Rareness is also a rule.
Religious rules also determine values too.
Then what are the natural rules (or universal rules) for human values, or general values for all beings?
Being truthful is a value and made it one's nature or a habit should give one a value.
In Buddhism, being truthful consistently is the only path to enlightenment.
Rahulovada Sutta clarifies it all: (Rahula was the Buddha's only son.)
Having turned the water dipper upside down, the Blessed One said to Ven. Rahula, "Rahula, do you see how this water dipper is turned upside down?"
"Rahula, whatever there is of a contemplative in anyone who feels no shame at telling a deliberate lie is turned upside down just like that."
Being a person empty of truth is like an empty water dipper.
Sometimes we have to deal with a dilemma. A story of a lion and a hare - The lion wanted to eat the hare so he asked the hare whether his mouth smell bad. The hare knowing the lion wanted to eat it discerned that if it said the lion mouth smelled bad, it would make the lion angry and would kill it. If it said the mouth didn't smell bad, that would be a lie and would make lion angry for being lied to and thus, would kill it. So the hare was forced to say it had a cold so couldn't smell anything. Although it is still a lie, it's acceptable because of the situation. The hare had to be cleaver and outsmart the lion.