Suggestion 1: First the hard but accurate way: Find out your cost for electricity for the firing. This is electricity cost.  Load and unload a kiln with a timer to get an estimate of the time it takes to physically do the work. Double this to accommodate the extra cleaning reloading etc that is involved with firing other people's work.  Assume this is moderately skilled labor work which you would pay someone about $7 per hour to do for you. Multiply the time by the money. This is labor cost. The cost of one bag of kiln wash (you will have to rewash your shelves more frequently if you are firing others work-glaze accidents etc. this is materials cost. add electricity, labor, and materials.  divide this by the cubic inches of firing space in your kiln. this gives you a price per cubic inch of work.

Easy but inaccurate way: have a small-medium-large price scheme and have each person pay per piece according to an eyeball estimate of the piece. (you need to have a chart for yourself that will estimate how many SMALL things will fit into your kiln in the various combinations-i.e... 20 small plus 3 medium plus three large will fit in the kiln, etc)

Suggestion 2:  I charge $2.50/pound for each firing. It seems to work out okay for both the student and for me.

Suggestion 3: I occasionally fire for others at my home studio. I made up a contract that lists the costs and the charge if faulty glaze or construction makes a mess of my kiln, which is an incentive to be careful. It might be good to state a rush order charge as well. I charge by the shelf at 6"H x 16"D (minus shelf support space). The customer may use 1/4 shelf, 1/2, or whole. If 6-12" high space is needed, the charge is 2 x the 6"H. price; 18"H. is 3x. (kiln is 20" deep). To figure the price, I considered the time it takes to schedule receiving and customer pickup, loading & unloading the kiln, firing time, cost of electricity, plus wear on my kiln. Examples: full bisque kiln is $36, or 3 shelves x $12 each; one 12"H. x 6"D. piece = 1/4 shelf x 2 high = $6.

Suggestion 4: The going rate around here seems to be 1 cent/cubic inch for low fire (bisque or earthenware). More if they will be using your glazes.

Suggestion 5: I wrestled with this for a long time, and here is what I charge for custom firing. I looked at the cost of electricity. wear on the kiln, etc, and most
importantly, that I have invested in the kilns and need an adequate return. 
For the 10 cu ft electric kiln, which we fire to cone 04, the cost is $50/kiln.
That comes out to $.035/cu in (or .04 rounded up). For the 18 cu ft gas kiln, it is $200/kiln. That comes out to $.08/cu in. The major difference is that I have to be there to fire the gas, while the electric I can set and forget. People have to use our clay, or bring us the label or description in order for us to put it in the gas kiln. The cost also includes use of our glazes, if they wish.

Note: Most of the above were submitted in 2000 before electricity rates went up most places.  You may want to make the appropriate adjustments.  One of our past tips talks about how to calculate the cost of firing your electric kiln.