in the commercial world people usually mark things up 3x cost

1=cost
1=overhead
1-profit

in the case of photofinishing it might be a little more
because waste disposal is included in the overhead.

if you have a mini lab near you, or a school, or a household waste recovery center ... you might
see if you can bring your photo waste to them ( and figure out what to do
with your wash water ) ... and have everything recorded so if / when people
who aren't your clients / friends knock on your door and wonder what you are doing
with your "tailings" you can show them exactly what you are doing.
where i used to live people would be fined lots of $$ for not paying attention to this sort of thing.

good luck with your business!

- john