The United States purchased Alaska from the Russian Empire on March 30, 1867, for 7.2 million U.S. dollars at approximately two cents per acre ($4.74/km2). The actual transfer of sovereignty took place on the 6th October 1867.
It was estimated that the amount of timber in Alaska is more then what USA is paying to Russia for Alaska. Now the question is why Russia was ready to sell at such a petty sum. The answer is managing a remote place i.e. Alaska was a pain for Russia and there was lot of discomfort between native people and Russian merchants. The massive earth quake (Reactor Scale: 9.2) of March 27, 1964 in which 133 people was killed and several villages and portions of large coastal communities was destroyed made the matter even worse. Which forced Russia to get rid of such a territory.