(thanks)
GOOD Galleries take half, yes.
Lesser galleries take 40%, or even %30.
The Good galleries have a huge list of collectors.
They have museum contacts.
They Advertise in Newspapers and trade publications.
They get reviewers to review.
They Charge more and take more.
If you have a contract with an Art gallery, they expect all sales to go through them.
If you sell a piece "out of your studio", you'll be sued and never get into a good gallery, again (unless you're super hot-shit)
It's like selling your home though a realtor. You can't just sell it to your friend, and not tell the realtor.