Ideally under €40
Will push it to €50 if it's really good.
Money, vouchers, clothes, books, DVDs, CDs, playstation/computer/xbox games, aftershave.
For my 14yo cousin we usually give him money or a voucher 'cause that way he can get what he wants with it.
Normally his parents will get him a games console so by giving him money or a voucher he can go and get whatever games he might want.
It all depends on the boy in question though, what is he into?
At that age they love their brands so a branded hoodie would go down well. Last year we got for our 13yr old as a stocking filler a Ben Sherman watch and wallet from argos it was half price approx 26e. He still uses both so it was a good buy.
Its extreely difficult to buy anything for teens, boys being harder (in my experience) than girls.
None of the teenagers I currently know - or even knew when my elder two were that age were into brand name goods, nor is my 12 year old. In fact my hated anything with a well know brand on it. .. .. but I am told regularly that teenagers love their brands (certainly it must have some truth in it .. I know from my mother than my brothers kids will only wear brands).
I would guess aftershave/shower gel type sets may go down well, as a nod to a 'grown up' present.
Find out what sort of movies he likes, make up a basket with some cheep dvd's in the genre he likes, along with bags of popcorn and chocolate and some cans of coke?
A lot of them at that age love music, but you could easily go totally wrong with the genre here, getting AC/DC for a JLS fan simply would not do!! So if your going for music vouchers for the likes of hmv or such like would be a better route.
And in a lot of ways they are still kids at that age too ... so giant space hoppers, skateboards, new football boots could also work.
Its a nightmare isn't it - teenagers :-)