It is possible to determine the vale in SP in a formulaic manner if you keep a few things in mind.
I stuff mine with cracked corn which I buy 50# at a time (we also use it to feed the birds during the winter, along with sunflower seeds), as it is the least expensive of the permissible fillers.
1 - Depending on where and when you buy it, the cost is around 20 cents a pound. Weigh a pound of finished products and multiply by .2 and divide by the number of packets to get the filler value. The weight of the bindings and cloth is negligible.
2 - One gets 36 packets per square yard of material, not counting wastage. I use the cheap stuff, that never costs more than $2 per yard, often $1 to $1.50.
3 Determine how many you can make in a steady 50 minutes to an hour of work (I never work steady, but rather cut the material during non-Tivo commercials and put the packets together while watching TV. It does not matter how long it actually takes, because I know how many I would cut and make in around an hour straight through).
Once you have your material and labor costs per unit, the rest is easy.
I also bring them in Plastic storage, from kitty litter, pretzel, and rice containers. James found out that if corn is stored in cloth inside cardboard, all one has to show the next month is fat rodents. I bring back my empties and refill them.
I can do 400 packets for around 2000SP. The game went through 200 before main mod last Event, so I plan on bringing double that this time. There is a lot of SP to be earned during commercials.