(Oh look, a post with actual Angel generated content!)
I was lucky. I started writing in the 80s, sending the laser printed copies of my first three chapters and synopsis off to New York Publishers. I accumulated an impressive array of rejection ships.
But at base, I knew, KNEW that I would not pay anything out of pocket after the initial mailing, not beyond postage for a contract.
Those coming up in the brave new world of publishing don't have that knowledge. I am frequently asked "How much will it cost me to be published with your press?" My response is always, "Nothing. You pay for NOTHING except discounted physical copies of your books, after the contributor copy." And then I lose my temper and rant a bit about predatory vanity presses.
It all comes back to Yog's Law.
In the early days of the net, author James D. MacDonald used the nickname "Yog Sysop," an obvious play on the Lovecraftian Yog-Sothoth. When he was asked how much he paid to have his books published, he had a rant, and started a one-man war on vanity presses.
He coined Yog's Law: "Money should flow toward the author."
Any professional will quote this at you.
John Scalzi puts it more bluntly: "Fuck you, pay me."
(some of us call the Scalzi's Corollary)
None of this applies to self-publishing. You're going to be out-of-pocket for editing and layout and cover art and promo supplies. But even the smallest of micropresses should charge you nothing.
A word about the physical copies:
You should always receive one free copy of your book, at minimum.
Beyond that, you will be expected to pay for the other copies you plan to give as gifts or resell. You should get these at a discount, specified in your contract. The difference between the price you pay and the price you sell at is considered your royalty payment.
If your publisher demands a reading fee, a cover art fee, an editing or any kind of fee, RUN. This is a vanity press. Alas, even venerable Harlequin has fallen into this with their Horizons imprint.
(Buying the cover art after the book is out of print is another topic. Artists have to be paid too)
If you are not getting statements or checks, (preferably both) at the agreed-upon interval, complain, Loudly.
Also, beware of "movie options." There is a company that has you pay $750 to get a $500 payment and have your book optioned by a very minor, local film studio. This does not guarantee your book will be a movie and you may be out $250 for nothing, or for an amateur bit of theater perpetrated by someone who knows nothing of lighting, SPFX or pacing.
Money flows to you, the author.