I would think that if it went to small -- and this is really small--claims court, you would win.

Anybody who knows anything about folk music--and they should, since they thought they were going to a folk concert--should realize, as noted earlier, that protest is often a feature. Suppose they had been bankers, and Magpie had sung "Pretty Boy Floyd"-- "...some with a fountain pen". They might well not have liked that either.

So it boils down to:

1) Are you willing to waste the time it would take in small claims court--if they in fact pursue it that far?

2) Is their goodwill--at least salvaging a bit of it--worth the $40 to you?   Particularly are they likely to be able to poison the well, so future house concerts don't do that well? I'd think that unlikely--since mostly future attendees will know what they are likely to get.
d