Normally £5, up to £20 for people I line manage. Made the mistake of topping up a meagre collection for a difficult former member of my team. Their final line manager said they criticised every gift they were given. Wouldn't do it again.
Very poor for line managers not to contribute to their own team's collections, although I have seen it happen. Often by people who expect to receive generous gifts from their teams. One senior manager in particular was openly speculating what he would get based on the fact that the CEOs PA had received a good number of expensive gifts "and she was just a secretary". What he failed to take into account was that everyone knew her, she was helpful, kind and popular. He never donated to anything, often feuded with others and micromanaged his team. His gift was a paperback book about his hobby.