Perhaps the biggest cost of war is losing our loved ones on the front lines of battle. For that matter it can also be a high cost of war when innocent men, women and children get killed in the crossfire. So, Bernie Saunders had a point with this statement losing loved ones is the true cost of war that can never be replaced.
This is an oversimplified argument meant to emotionally manipulative listeners. The horrors of war are not just as simple as losing family members. Of course, losing children in war is a terrible thing, but the real cost of war comes from the combination of all the lives lost, not just from the people you know who are killed.
I think Bernie Sanders is correct that losing children to the front lines is a cost of war, but I don't think it's the true and only cost. There are a lot of costs of war and losing children is probably the saddest. I think Sanders oversimplified war when he stated that losing children is the true cost.
The main cost is the perpetuation of wars that take many lives, not just MY children's. Each life is worth the same and it's not worse because it's someone I happen to know. Sure it might feel worse because it hits closer to home, but it really is no different.
As most have already previously stated, there are many costs of war, and there really isn't a central or "true" cost that outweighs all others. Death is one of these penalties. Among this cost is the economic cost, loss of infrastructure, and radical cultural changes for better or worse, just to name a few. The argument can be made for any of these costs that they are THE "true" cost of war.