We try and keep 'extras' to a minimum, but dc are allowed one paying club (approx £70-£100 each per term) and then just whole class trips which cost from 0 - £245 for a residential. DS has had one residential per year from y4.
for DS - pretty much all uniform from second hand shop for around £150.
After school care free from 8-8.30, and 4.15 - 5pm. About £2 for 5-6 pm. All of which includes snack and drink.
No text book fees, but we provide all stationary.
for DD - second hand shop uniform £125, sports kit new from shop £250 
After school care free 8-8.30am, £10 4-5pm, £8 5-6pm.
Provide own snack if required.
Again no text book fees, but own stationary (particular brands specified by school) used down to scissors and glue sticks.
The uniform is of course more or less a one-off expense with top-ups as you go.
Other extras are music exam fees £30 ish, teachers presents £10x2 (Xmas and July), Music books £2-£10,
As you can see, costs vary wildly between schools so it pays to know a parent well enough to ask about costs at a particular school.
Ideally I would dress dd up as a boy and send her to ds's school!!!! Odd that they vary as much though, as the schools are kind of affiliated as sister schools.
Depends also if you feel the need to 'keep-up-with-the-Jones'. Then you get into having a 4x4, holidays / skiing, huge birthday parties, designer clothes, horses, personal trainers etc etc etc. 
hth.