Mine are at a "cheap" private school in the Southeast. This is my costs for my year 8 child so far, I also have another one in year seven.
£1500k per month on fees currently (with 20% scholarship)
£30 a month Standing Order to my daughter's bank account for snacks and charity days etc
£750 a year on school uniform if I bought everything brand new, probably about a third of that if you can get everything second hand
£200-£300 a term on extra curriculars
An average of £2k a year on trips (less now but looking at the five year plan, they increase later).
Tablet for school (£300) now, I'm expecting to buy another one before she finishes.
Laptop for school, she has a Chromebook now but I will upgrade in year 10.
School events, charity days, Xmas fayres etc £300 a year.
Music tuition £150 per child per term
Also we have to pay for exam entries.
There are also the hidden costs of socialising and demands. Richer families come with more expensive tastes, north face coats for school, certain shoes, ugg boots, Sol de Janeiro, spark NK make up, trips for boba teas, starbucks etc. I can't afford half of it and my youngest is forever giving me grief for it.
I don't regret my choices, but I did underestimate the extras.