I don't know about the total cost, but buying in bulk at Lowes or Home Depot will probably keep the cost down -

You can probably pack this in a zip-lock of some kind that would double as a water carrier (although I don't know how long a zip-lock will hold up in a scout's cargo pocket...

- 55 gallon trash bag for emergency shelter, etc. Buy a box.
- some cordage, heaver than string, lighter than rope. Buy a roll and cut lengths for each scout.
- sheet of heavy duty foil, improvise pot to heat water. Buy a roll and give each scout a piece to fold.
- some kind of age appropriate fire starter, water-proof matches, mini-Bic?
- some kind of tender - buy a big box of wax/sawdust fireplace starters.

If there is money left over, buy a bunch of Calume sticks and put one in each kit. Can be ordered bulk on Amazon or maybe from a local supplier.

Just having the trash bag can stave off hypothermia and save a life.