The thing is, it depends a lot on your dcs experience and expectations.
I genuinely don't think any of my dc would ever dream of "going to Costa with their mates after school" - ok, partially because they already have quite a walk home and there's no Costa or similar anywhere near that walk mainly because it's just not something they've ever been used to - It's not something dh or I have ever done, and I just don't think it would occur to them, any more than going into a betting shop or something. It's not that they wouldn't be allowed, it just would never cross their minds. My dc go to friends house, or the park, or friends come here after school if they want to be with their friends. It's not that there is anything wrong with going to Costa, but if you've never done it, then it just doesn't cross your mind. Whereas, if they have been used to going in on a weekly basis with you, then, if you don't give them enough to fund it, you are, in effect 'depriving' them of something they see as a normal habit. Does that make sense ? 
Same applies for other things people chuck in on threads like this .... how do they afford the cinema / a magazine / going into town with mates on a Saturday / or whatever - you need to give them enough to cover this, if that's what thy see as normal weekly activities, but if it's an occasional, twice a year type thing, so they've not been used to doing them or getting them, then less money is fine as they've plenty of time to develop a savings pot.