Louisiana Purchase was $15 million in 1803.

Alaska Purchase was $7.2 million in 1867.

Inflation puts $15 million in 1803 as being $19.6 million in 1867. So that puts Alaska purchase as 36.6% the price of Louisiana purchase. Honestly I can see why it was seen as a folly (though yes Louisiana as seen as a bargain.)

www.in2013dollars.com/us/inflation