How do you come up with a ratio that you consider a fair trade?

I'm really not sure how I'd personally set a metric to decide it. I could go with the stat that one barrel of oil is equivalent to 25,000 hours of human labor. That means each barrel is worth 12.5 years of labor at 40 hours per week. That seems outrageous though - off hand I don't know how many barrels would be used during the flight but it would have to be replacing way more than several engineers working for several years.