We have been looking at this for costing university. Making sure we have enough and to give him a sensible amount.
We upped our DS (nearly 18) monthly pocket money to £200 but have stopped funding lots of things. He has been on the new budget for a year now and it has worked really well.
He gets very basic clothes from us - we won't see him naked and shoeless but if he wants more than very basic t-shirt, jeans and one pair of shoes/trainers he buys it himself. We buy school essentials books, stationary and clothes (he has to wear all black rather than uniform). We bought him a nice outfit to wear to a wedding because we wanted him to look nice and paid for a haircut - but hair is normally something we'd expect him to have to cover.
We make sure that there is food he likes in the house so he can take a packed lunch to school and he gets £20 a month on his school canteen account for drinks but if he wants to go to the local cafe and buy lunch that is on him. (He is only in school for 3 lunchtimes). He sometimes takes a microwave pasta to school for lunch as they have microwaves that students can use.
We pay for his hobby which is £180 a term. We pay basic SIM on a family account but his phone is quite old now (still an iPhone but an older model). He has enough data to last because we share and pass our unused data to him.
It has worked really well - he likes having the money in his pocket monthly like that and is learning budgeting - he likes to go and see bands and to the theatre (National Theatre £5 tickets for under 18s has done him well).
It has really taught him bargain hunting He raids charity shops and Vinted for clothes and mixes that with the basics we're willing to provide. TKMaxx and Offcuts for shoes - DMs from Offcuts for £40, 'cool' Puma trainers for £25.
Unfortunately he has not been able to get a job - he has tried but no one is employing kids his age round here. His friends who do have jobs all work for family members or were very lucky.
We live in London so free bus travel and tubes are reduced so travel not too expensive and he has the choice of walking or bussing to school.
It has really taught him a lot about budgeting, how much he costs us and to prioritise what is important to him.
Monthly
pocket money £200
phone £10
hobby £60
school canteen account £20
average of clothes/school essentials (clothes, books, etc/hair etc £50)
Total - £340