Alex Tabarrok at Marginal Revolution summarizes DeLong on the Civil War. Direct per capita costs for north were $140. Cost of the north buying and releasing the slaves would have been $90 per capita. Direct and indirect costs to the liberated land, the south, totaled $490 per capita.
Fast forward to 2005:
Cost to give every Iraqi citizen 5 years income (based on pre-war GDP): $189.6 billion. I suspect that even the much feared and overhyped Sunni population might settle their asses down and run the terrorists out themselves if we just paid them well enough.
Cost to buy off all Syrian troops in Lebanon with $1 million each. $16 billion.