figure on pool time @$100. for an hr.--New York City rates! thats what it can be..some pools charge per group at that rate,some by person ,at that rate!
then figure on 2 hrs cost to the store for the instructor time(pool time/meet and greet/travel/equipment/insurance/gas for vehicle/) @$50. per hr(store is paying instructor on books-so workmans comp,social security,benefits,)the instructor may get $15.-$20. per hour of this $50.
So now we are up to $200. this is for a person/persons, that were originally to complete confined water class with a group(lower costs to facility)and for whatever reason could not make schedule.
I know it sounds harsh, but for a facility that has to rent a pool and then send an instructor there , these are close to the actual costs.If the facility has its own inhouse pool, and an instructor in house paid hourly on a 40 hr a week basis,then there may be more flexibility in what is charged.
It is a business and expenses have to be charged to survive.Did these people purchase their gear at the facility or went online for it? That can have a bearing on things also.If the facility made gear sales to them I am sure the service may have been better. Not to play devils advocate here or to say OP is in the wrong.Just giving different possible view of things.