"... low opportunity cost of fighting ..." - The Economics of War by William Hynes"Opportunity cost" can be roughly translated as opportunity lost. You only have so much time. You can spend your time working or playing or sleeping. If you are doing one activity (or inactivity), you can't do the others. To make the most of your available time, you should choose wisely. If you are trying to provide for yourself, and none of your available options are likely to provide much of anything, then the cost of not doing them is low. In other words, if someone is willing to pay you $10 a day to carry a gun around, and there are no other jobs available, carrying a gun looks like a pretty good job.
This is from a short (six page) paper on Academia dot edu.