A good rule of thumb in business is the selling price is about 3x the direct manufacturing cost. There are all sorts of businesses that don't adhere to this model, but the majority do.
If you hire someone at $12/hour (+$6 taxes & benefits) to due lab work, and it takes them an hour and uses $2 of chems & paper to do a roll of film then you need to price the service at around $60 for an individually processed roll of film.
Obviously, as a business this won't fly.
Develop 5 rolls of film at a time in a large tank, supply digital proofs from a mini-lab printer or build some sort of quick-to-operate contacting machine and a dry-to-dry processor and you can get the labor down to 1/5th hour per film and the chemicals down to $1. Now the price becomes $11 with a charge for handling and shipping - it is best to charge these at close to cost.
On that basis, to keep a worker busy on this one business, you will need to process around 30 rolls of film per day (figure 20 some days, 40 others) and pay the occasional overtime when you get a surge of orders.
Now you can use the money left to pay for advertising [possibly the largest indirect expense relative to sales], marketing [schmoozing], rent, janitorial, laboratory equipment, shortage & scrap, business insurance, telephone, utilities, the BBB, the accountant, company taxes ... and maybe have a few thousand left at the end of the year to pay yourself, the owner/operator [notice that everyone but you is making decent money]. Obviously, you need to have some other source of money to support yourself through this start-up period.
If you can grow the business to 3 or 4 workers it can begin to pay you a living. You will never work so hard as you will at this - don't think you would be making money on the sweating back of the working class.
Or you can pay yourself the $12/hour and try to run the company evenings and weekends - not recommended.
Or you can do it as a hobby-business, processing film in the evenings and on weekends in your own basement, pricing the service at what the market will accept, and count on word-of-mouth for most of your advertising. You will probably make more money flipping hamburgers, however.