We actually negotiated a contract with DC1 when she turned 14 and was no longer happy to go shopping with me for clothes in mid-budget shops 😳🤣
She gets a monthly allowance. The contract states exactly what we pay for and what she pays for.
We increased the amount when she started college at 16 (partly because she started to commute into the nearest city and partly because she chose an unpaid (but long term vocationally sound) academic route to reach goals she'd thought through sensibly while a lot of her friends chose paid apprenticeships).
We've followed the same model for dc2 (boy) with a couple of agreed adaptions seen as fair by both dc1 and 2.
DD had stopped growing shoe size and height wise at 14 but I find a well fitting bra important so I said I'll pay for 3 really decent new bras per year on top of her allowance 🤣 DS on the other hand is (unsurprisingly for a boy) still growing so instead of bras I pay for two new pairs of mid range shoes each year (one in autumn, one in spring) and one pair of football boots. If he wants anything above the price I'm prepared to pay he tops up - so I'll pay up to say £40 but if he wants to spend £80 that's up to him, he just has to add his £40 to mine.
Clothing wise their allowances pay for everything else - they both favour sports brands but shop thriftily on Vinted and in sales.
We pay for annual travel cards.
We pay for driving lessons (we don't give them lumps of cash in case they want driving lessons, but pay directly for lessons taken and tests).
We pay for basic phones and basic data packages - they want more, we'll provide the cost of the basic and they can add to it.
We pay for them to go on holiday with us and on school trips - they pay for holidays with mates or boy/ girlfriends.
They buy whatever birthday and Christmas presents they want to give anyone.
They pay for their social lives.
Snacks - I buy some communal snacks which if anyone opens they have to eat in the communal areas and offer around. If they want a private stash that's fine but they buy it (if I'm doing a shop anyway I'll buy it but they transfer the exact cost to me, and I won't stop at a shop just to buy a snack on demand - they have bicycles...)
DD works (tutoring and babysitting through a proper agency, but she has flexibility around her studies) and DS doesn't so far, though to be fair he's only 15 and although there are lots of jobs for 16+ around us there's very little for 15 year olds outside some very specific summer holiday only jobs.
It works well with very little discussion or debate.
We never, ever, give or lend cash or buy random items (unless it's a birthday or Christmas present, in which case they get what we would spend on a present if they prefer cash).
DS (15) gets 80€ per month, which is about £65 or £70, and DD (17) gets 100€. I actually think that's fair for all clothes, snacks and socialising etc.