So, by now you're probably chomping at the bit for some examples of how the numbers work in publisher. A traditional publisher may offer a royalty that is somewhere from 10% to 15% of the net price. The net price is what the publisher gets for the book. The publisher sells the book at a discount to bookstores. This varies and used to be about 40% but domination by just a few big book chains has forced it closer to 50%. That means if the price on the cover is $20 you may get 10% of $10 or $1 per book. You might get as much as 15% of $12, which is $1.80 but I wouldn't count on it. Remember that royalties on foreign sales are likely to be half of domestic and book club sales are often at a special discount (all of these things are technically negotiable but it might be hard to get a publisher to budge from their standard terms).
Now, a publisher may well be happy with a first run of 6,000 to 7,000 copies that sells close to that number. So an advance of around $10,000 is quite common. You may get a check for that on signing the contract or staged over the delivery of the manuscript (one third on sgning, one third at halfway, final third on final mnanuscript approval).
Now, I am pretty sure you can get on the New York Times Best Seller list with one week sales of 10,000 or more [but would be happy of someone could correct me or expand on that]. So a book that is a "best seller" may sell only 30,000 copies and I am under the impression that sales of that level for a literary novel are considered good. In that case the author may earn $40,000 or so. Not bad, but relate that to the time taken. A lawyer or other professional may well bill at $300 an hour or more. Earn $30,000 from a book and you are looking at less than three weeks of billable hours. And the publisher will likely tell you this is a very good outcome.