Right now I am doing the same thing for my scout troop, in fact we are working on our hammocks tonight. I got the total cost to about $28/hammock.
Here are my parts list:
Sources:
Hammock Bodies
Webbing (Mule Tape - John Sawyer)
Rings
Rope Amsteel
Ridgeline (Amsteel) (Same As Above)
Tarps (6x8) (On Sale Locally)
Stakes
Bookmarks