ok lets throw some real world numbers at it (some are guesses, some are actual numbers we derived). I was park operations supervisor of a small amusement park.
any given restroom was supposed to be checked every 30 min. Clean up any obvious mess, change trash can if full, restock any supplies that may have run out.
So call it 5 min per restroom per hour, sometimes its 2 min, sometimes its 10.
Average restroom traffic for us was around 20 /hr
so a $8/hr employee, average 5 min $0.66 /20 customers $3.3c each
about 30 min at the end of the night, averaged across all customers $4.00 labor
$0.03 each
TP rolls, we went through 2-3 per shift per restroom, so call that $1.50 for 160 customers or about $0.10 each
Call it $0.15-$0.20 per customer. Of course that amounts to about $3 per hour and if you have higher customer volume you will have more messes and higher supply consumption. Water usage is not insignificant for the day. Call it another $0.01 per customer.
Wear/tear, replace seats every 6 mo or so, the occasional bit of tank hardware, and a maintenance guy who probably makes far more per hour but might interact with a bathroom a couple hours per month. Not so bad lets call him parts and labor $0.02 per customer.
Not hard to imagine it costing $0.25-$0.30 per customer (estimating high)
So apply that to a fast food restaurant open for 12 hours at those numbers averaging 20/hour and you have about $80/shift
About $2,400/month.
More volume increases that.
So even though the individual customer is minimal, ramping up volume can add some hefty numbers to a month.
We did a bunch of studies on stuff like this looking for efficiency issues in the park related to non revenue generating tasks.