What is a wonder oven? Works like a power-less crock pot. It does require initial heating or food but is designed to reduce the amount of fuel needed to cook food. It can also keep prepared food hot and cold if necessary.
On average what does it cost to make one? Around $20. Many people will have 100% cotton fabric at home in different forms that can be used which will cut down on the cost. The cost is associated with purchasing the fabric and also the polystyrene beads. The beads can be purchased in many places but locally you can buy it from Kmart or Amazon. I personally bought a small bean bag filled with these beads from Walmart and cannibalized it.
Now since I have your interest what is the next step? I have made 5 of these. I would be more than willing to help you and even your Ward do a mass production of these if there was interest. I have the pattern for the top and bottom pieces pre-cut and would be happy to share this with you. (Having he exact pattern needed is a pain to get traced out).
Wonder box construction instructions? iwillprepare.com
Please let me know if you need help putting one together or if I can help your Ward mass produce these.
Hope this helps.