I think I'm reasonably generous but £15 sounds a lot of money to me, particularly as several people are contributing. I too think it's cheeky to expect everyone to give that sort of amount - everybody's financial circumstances are different.
And for the OP, who has just joined the group and doesn't really know the person, I think it should be left to her to decide whether she wants to contribute a couple of pounds or not.
Each Christmas my choir makes a collection for a present for our musical director. There are about 20 of us and we usually contribute £2 each (and if someone wants to put in a little more, they can just place it in the envelope without drawing attention to it).