It's about as fair as when Activity Directors who pay about $50-$60 for a gig and generally book the same folks once or twice a year expect you to come do a free program ("audition") to get those one or two bookings. If they don't decide to hire you, they've gotten a free program. If they do, you'll essentially have done your paid gig(s) for half (or 2/3) the $.

But I guess "fair" isn't the question. It's "What will the market bear?"

If I thought the gig was worth enough (exposure/publicity, $, etc.), I'd keep both dates open (especially if the rain date wasn't a "prime time" spot like St. Patrick's).

Genie