I have explained this several times before.
Assuming it is a major publishing house and you have an agent, you can expect this.
The price of a book is based on 6 times the cost of producing the book. Basically it comes to around $24.95 - something in that range for about a 350 page book. Your royalties would be 10% or $2.49 less 15% for your agent or about $2.12. 10% is a standard writer's contract.
The average first print run for a first time author is about 15,000 copies. Much less makes it not profitable to print. Assuming your agent is a good one, he or she will be able to negotiate you a 50% advance on your total commission. Or about $15,783.
In addition to the 15% commission, your agent is legally entitled to bill you for incidentals like making copies of your manuscript, postage, long distance calls, etc. -- all things they have to do in order to sell your book. They may not charge you for a reading fee, but they may charge you for these other things. And figure that at about 7 cents a copy, every time they duplicate a 350 page manuscript to send to a publisher to read, it costs you about $25.00.
If your book sells 7,501 copies, you start collecting royalties at the rate of $2.12 for each book you sell. If it sells less than 7,500 you will likely owe the publisher a refund of part of the advance. If your books end up on the bargain tables at the major bookstores, your royalty is ZERO.
If your agent and publisher negotiate with large wholesalers like Costco of BJ Warehouse, they will sell your books at about 55% of original cover. That lowers your royalties significantly. Down to around $1.37. However, your book will have a much larger base for people to purchase it. So there is some advantage to that.
Most first time authors take their advance and spend it on merchandising and marketing so they will guarantee higher sales. Your publisher may or may not kick in some promotion dollars - depends on your agent. Just so you know - to get your books on those tables inside the doors at Barnes and Noble will cost you about 1 dollar a book to Barnes and Noble. That is all paid advertising. Publishers rarely go all out on first time authors unless the book is very exceptional. The idea is to promote your book so the entire first run sells and the publisher prints a second, third etc. run. Your agent can negotiate a better deal for you then. But you have to be willing to gamble and spend money to make money.
On the other hand, if your book is being published by a smaller publisher (the more likely scenario) You can expect zero advance and a quarterly royalty check of 10% of your sales.
If you self publish, you pay to have your book printed and then you are on your own to sell it, promote it, etc.
So basically, the answer in short is that you will not see very much money on your average first book deal. In America today, traditional publishers are putting out 150 thousand books a year. And self publishers are probably putting out more than triple that. You are up against a lot of competition and a publisher is more likely to spend his advertising dollars on the latest Dean Koontz or James Patterson than on a newcomer.
That is basically how it works.
They're, Their, There - Three Different Words.
Careful or you may wind up in my next novel.
Pax - C