It really depends on how much you want them to 'keep up' and how social they are.
Day to day it's not too bad. My 14yo ds gets £15pw lunch money.
Phone £8pm.
Gym member £20pm.
Pocket money £20pm but gets extra for things like McDonald's, cinema.
Football season ticket was about £150.
But things like activities and eating out gets a lot more expensive as you're into adults prices.
Clothes are our biggest expense. For example his school shoes (black Nike 95s) were £180, he said 'everyone has them' and it thought that can't possibly be true but I looked round his friendship group, asked parents, and they do!
Some of the things he has...Nike tech joggers £110, CP company hoodie £300, Fred Perry Polo shirt £75 and he needs more than one t-shirt! Basic Nike Miler or Under Armour t-shirts are £30-£35 each.
Obviously you do not need to buy these things but risk them feeling left out if they don't have the right stuff. And yes you can get them for birthdays but it's also our role as parents to provide clothes.
Xmas D's has asked for a PC accessory that's £120 and a jumper at £70. It's these kinds of things that make life very expensive as the cheaper alternatives just won't cut it.