William H. Seward, secretary of state under Abraham Lincoln and Andrew Johnson, negotiated the purchase of Alaska from Russia. His original offer was for five million dollars, and while the Russian Minister was relaying that figure to the tsar Seward got permission from the Cabinet to offer up to seven million dollars. The final figure agreed upon on March 30, 1867 was 7.2 million dollars, equivalent to 2.5 cents per acre. On October 18 at Sitka, the Russian flag was lowered and the American flag raised over "Seward's Icebox." Critics attacked Seward for the having conducted the deal in secret, and for having purchased what seemed to be a desolate wasteland. In the late 1890s, however, gold was discovered in Alaska, boosting its population and value as a U.S. possession. The area became a territory in 1912 and in 1946 residents approved statehood. A state constitution was adopted in 1955, and Alaska became the 49th state on January 3, 1959.

Sources: http://www.everythingalaska.com/eta.sfy.html and http://memory.loc.gov/ammem/today/mar30.html