You get x amount per child.
This covers rent, cleaning, toilet paper, soap etc, training and salaries. internet, insurance, admin, stationary, furniture, repairs, maintenance.caretaker, equipment. Additional training for fire marshalls, asbestos training, medical needs, Sendco etc, and supply cover.
Teachers £31.650 starting rate
Access to SALT, any additional needs, assessments, Ed P, maintaining your records and reporting to the LA. Ed P can be over £80 to £120 an hour
Bin collections, paper and medical supplies, electric; rates may apply depending on status. Safeguarding, H&S, Legal services, HR, wage slips, maternity and paternity insurance or paid if not insured, pensions, legionnaires, PAT, deep cleans, advertising a teaching post is about £1000 a time, etc.
That all comes out any payments received.
Funds credited for FSM is at lower rate than the cost. Schools are expected to subsidise costs from their budget. The costs also cover staffing and training for MDS if you employ them rather than having a private provider. There are costs for free fruit and milk too.
There are costs to ensure you correctly record who can access FSMs even if everyone in your class is entitled to FSM.