Top end galleries can take as much as 60%. Your average gallery will take 50%. At Helios we take considerably less — but selling art isn’t a core part of our business (teaching is) so having excellent work in our window has huge benefits for us. We also do very little market art — which is something you should expect if you are giving up 1/2 the price of the work.
Make sure you have a written agreement that covers things like shipping costs (possibly both ways) if the gallery isn’t local and liability if it gets broken.
Lastly – and what I think often impacts artists the most – you shouldn’t sell your work on your own for less than the gallery sells it.
Hope this helps.
Helios Kiln Glass Studio