With the cost of chemistry at a little more than $43 a kit (I'll assume the price is $40 for simplicity of demonstration), and with 120 film having roughly the same area as a 24 exp. roll of 35mm, and with one kit being able to do 120 rolls of 35mm 36 exp rolls (for the equivalent of 180 rolls of 24 exp. 35mm or rolls of 120), the cost comes out to $0.23 a roll, maybe $0.27 or 0.28 with the actual cost being $43. or 44. I'm going to say $0.30 for a roll of 120 to factor in stuff like water. This is with chemical replenishment every 20 rolls and a dump every 40, which gives 3 refils with a 4L tank and 13.5L kit. So, around 5c/foot of 35mm or 10c per foot of 70mm/medium format. The lowest I've seen a lab charge is $0.60 per foot of 70mm, so labs make a fair profit on it, anywhere from 2-6 times as I don't know the price they're spending on their C-41 chemistry. I'd say the cost is between 10 and 30c per foot of 70mm film or 5 and 15c per foot of 35mm
Regards,
~Karl Borowski