"A teens allowance should be enough to cover most regular activities, but not much more than that." (Parenting Teens with love and Logic, Cline and Fay, p. 253)
It could include
- school lunches
- movies or snacks with friends
- birthday presents for friends and relatives
- field trips
and might even include
- school supplies
Don't forget, they could or even should be earning some money on their own and some of their expenses should can be covered by that.
Parental; backbone tip: "Stick to the rule for yourself that "When it is gone it is gone!" No advances, loans, etc.