The Alaska Purchase? is a 
Russian event, which is provoked by the American decision Seward's Icebox. With this decision USA proposes to buy Alaska for the price of 70.000£
Seward's Icebox can be taken by 
USA, if they are at peace, have £70,000 in cash, have at least 1 prestige and have researched Nationalism and Imperialism. All of these conditions are almost inevitable to happen, so Russia will face this choice basically every game.
If Russia agrees to the proposal Russia will lose all cores on Alaska, USA will gain cores instead, and they will annex the territory. Of course USA will also lose 70.000£, which Russia will gain. On top of that all POPs in USA lose 1 consciousness.
Technically it is all cores in the continent of North America, so if Russia annexes provinces from e.g. 
Mexico those will be sold as well. It will not affect the prize though. An AI-controlled Russia will however almost never have conquered any provinces outside of Alaska.
If Russia denies the proposal the relation between the two countries will decrease by 100.
One has to weight the worth of the province up against the worth of 70.000£. The province is not worth a lot, but on the other hand Russia shouldn't be in money troubles at the time Nationalism and Imperialism has been invented.
An AI-controlled Russia will accept the proposal 80% of the time.