DS has been invited to his first high school birthday party (year 8). Towards the end of primary, there weren't really parties, just small get togethers with his closest friends - eg a movie or something - and quite often the parents would all do a quick whip round and someone would either liaise with the parent to buy a single larger gift or stick it all in an envelope for the birthday person.
Clearly I now know NONE of the parents to get a sense! 
I'm thinking we shove a tenner in a card? Does that sound right?
For context, we're in a bog standard middle class (not wealthy, definitely more on the "getting by") part of Surrey (on the London border). DS is at a large state school and his friends all seem to be roughly similar to us in terms of type of lifestyle etc.