I play in a band, and I also help to run a ceilidh series, so I see it from both sides.

When I'm booking bands for the ceilidhs we'll offer a percentage of the door with a guarantee. It's unfair to expect bands to travel long distances without knowing that they will at least cover their expenses. If a band is travelling a very long way we may agree to a higher guarantee. In most cases we end up paying them more than the guarantee so it becomes hypothetical.

As a performer, I'm usually happy to play for a percentage fee, provided I am confident that the event will be run and promoted properly. However I'll insist on a guarantee. Firstly, it means I know what I'll get for the gig - if I get more, so much the better, but if not I can't complain. Secondly, it incentivises the organiser to make an effort to promote the event properly - with no guarantee the financial risk lies entirely with the performer.

As Will says, the important thing is that the terms are clearly agreed beforehand. If the performer doesn't like the terms, he shouldn't take the gig. On the other hand, if you don't think you can make enough money to offer the performer at least a minimum fee, is it really fair to be putting the event on in the first place?