The following table is from a spreadsheet I put together analyzing the relative costs of the Alaska purchase ("Seward's Folly"), the Louisiana Purchase, and the approximately 10-year lease on Iraq Bush just signed on our behalf.

Territory · Year Acquired · Total cost (adjusted for inflation) · Area (sq. km.) · Cost/sq. km. · Cost relative to Iraq · · Population · Cost/person "liberated"
Louisiana Purchase · 1803 · $180,791,262 · 180,791,262 · $84 · 1/13,724 · · 97,000 · $1,864
Alaska · 1867 · $85,010,736 · 1,482,970 · $57 · 1/20,183 · · 32,000 · $2,657
Iraq · 2003 · $500,000,000,000 · 432,162 · $1,156,974 · 1 · · 24,683,313 · $20,257

The analysis deals only with money, of course, not the cost in blood; we got Alaska and the Lousiana Territories through peaceful negotiation. There's a lesson there, somewhere, on the cost-effectiveness of war. What price would Saddam Hussein have put on his little hellhole if we'd bothered to ask first?

(Thanks to FrankC for inspiring my curiosity by trying to argue that the cost of acquiring Iraq isn't "ruinous" compared to other things.)