The Ope Ope fruit and it's actual value

Does the fruit really worth 5 billion berries ?

The latest chapters explained the capability of the "ope ope no mi" and why is it so valuable that the navy put a huge prize on it.but what was really the actual prize of the fruit ?
Doflamingo said that the pirates who got this fruit are too stupid to sell it for 5 billion berries not knowing the true power of the "ope ope no mi". so I guess , the fruit worth 3-5 times the prize offered by the marines.

But what confused me is..

Why did doffy knew so much information about the fruit?

well, the captain of the pirates who got the "ope ope no mi" once said that there's a legendary doctor who ate the fruit , so obviously , it was the doctor who first discovered the power of the fruit and its ability to grant immortality. however , it was not mentioned if the doctor has been killed or someone has been granted immortality in exchange for the doctor's life and my best guess is , one of the elders of world government .

I highly doubt that the previous user of the fruit works for the world government , that's why doffy and roci knew about the fruit and my speculation is , if the doctor has been killed then doflamingo is the one who did it or someone from the highest rank of world government has been granted immortality as what i had mentioned above.