If you are doing all of the loading, remember to pay yourself for labor!
It's great that you're making the resource available, but a fair price for providing a firing service should be at least double what it actually costs you. Even at that price, it will still be a bargain for participants.
I rent part of my personal studio space to other potters. Renters are responsible for the cost of all of their own expendable materials and have assigned storage spaces for their work, tools, and materials. I have two firing fee structures, one if the renter does all of his/her own loading/unloading and another if I load their work. When I load, I base the cost on the height of the shelf required to accommodate their work and/or the percentage of the kiln space used, rather than weight or actual square inches. Ultimately, it is my judgment call. I strive to be fair and usually err on the side of charging on the low end of my estimate. I haven't had any complaints.
Finally, if renters want to use my glazes, they pay a fee per 25 lb bag of clay opened. This grew out of the system we use in the teaching studio where we buy the clay and sell it to the students with an up-charge that covers the cost of the studio glazes and underglazes.