to be asked to contribute?

I have never heard of such huge contribution to a simple leaving gift ! As has been said just explain you cannot afford more than £1 -£2. Even that would be too much for folk who are on only the basic old age pension and who want to join the group.

Yes, I think all those comments are dead right. £15 is a huge amount to spend on someone you have only met once. It's a shame you've been put in this awkward position though, the person asking should have realised that this really wasn't on. You are definitely not being unreasonable.

A token gift and a card will certainly suffice and I'm sure it will be appreciated by the lady.

Just as others have said. Explain that you are happy to make a token gesture to the leaving gift but can't afford £15 considering you barely know the lady.

£15 is a lot of money. Unless the others are all very good friends it seems a huge amount for a leaving gift. I would suggest a reply like the explanation you gave here and make a small donation or say you will give her a small gift yourself (nice bookmark or something).

It's an established group - an acquaintance told me her friend was in a bookclub and set it up for me. I get the impression some of them have known each other for a long time but I didn't get a chance to chat with many of them. The couple I sat next to were very welcoming and not clichey at all.

In your shoes I would offer a fiver and explain your situation just as you have on here. People who can afford such money are often simply unthinking and I doubt would take umbrage.

That sounds unreasonable to me. I wouldn’t contribute £15 to a gift in those circumstances. Is it a new group that you’ve just joined, have all the others known each other for longer?