ALASKA purchase by United states of America from Russia!!!!! When and Why?

"Alaska" is the treasury house of United States of America which is full of gold mine,diamond mines and rich in oil and petroleum .
BUT do you know ,someday it was not a part of America and was rulled by RUSSIA.

People of Russia called Alaska as the heaven of Russia.
But Russia sells Alaska to USA for $7.2million (45 crore 81 lakh ruppes) on 30th march 1867 .
Reason for selling of Alaska :

1. Although it was a fertile land but due to its vast area a big unit of soldiers had to stay for its security purpose.
2. Fear of war against USA & Britain

Alaska is the biggest state of USA.
~So what is the present value of Alaska?
Well,In the mid-1980s, Michael J. Boskin, a Stanford University economist, estimated that Alaska’s oil and gas reserves alone were worth at least $200 billion.
Then guess what Will be the total value of Alaska today???????