The kids were interested in the price difference between store-bought Rice Krispies Treats and homemade ones. OK, their father was interested in the price difference and got the kids involved.
First we bought a 40-bar box of Rice Krispies Treats for $7.49. We've never done this before; it was purchased strictly to find out how much each individually wrapped bar weighs, and the answer is around 21 grams, so that's 420 grams for $7.49 or $0.01783 per gram ... a little under two cents per gram.
Next we bought a four-stick package of President butter for $5.99, or about $1.50 a stick. The recipe calls for three tablespoons of butter, which is 3/7 of a stick or $0.64 worth of butter.
Then we bought a 10 ounce package of Jet-Puffed Miniature Marshmallows for $1.69. The recipe calls for one whole package.
Lastly we bought an 18 ounce box of Rice Krispies for $3.69. There are around 18.75 cups of Rice Krispies in an 18 ounce box and we can round down to 18 given the spillage that occurs when the kids make Rice Krispies Treats.
So the total cost per batch was $0.64 (butter) + $1.69 (marshmallows) + $1.23 (cereal) = $3.56. The yield of our first homemade batch was 431 grams, giving a per gram cost of $0.00825 ... it would be even lower if not for the spillage allowance.
The bottom line is the store-bought Rice Krispies Treats cost more than twice as much as homemade ones.
I will update the post with the yield that we get from future homemade batches to see how much variance there is batch to batch.
UPDATE: the second batch we made was 460 grams, giving an even lower per gram cost of $0.007391!