Trying to assess the potential earnings from selling a novel can be an exercise in futility. Advances can range from nothing into six figures, depending on the novel, the person negotiating the contract, and the publishing house. The "typical" advance quoted for first time novelists is generally between $2000 and $5000 but that is only the beginning of the payment process.
Publishers pay advances based on their sales expectations and the size of the print run. An advance against royalties is received upon acceptance, delivery of the manuscript or publication (or some combination thereof). This advance is considered the first royalty payment, and the author will not receive their next until they have sold enough books to "earn out" their advance.
A typical contract will pay the author between 5 and 10 percent of the sales of his or her book. To land a deal with a major publishing house, which will have better channels for distribution and marketing, you will need an agent. The agent works for a 15% commission and is more than just a salesperson. They have connections with editors and can get your book into the right hands. When a contract is offered, the agent will negotiate it and help you understand the details.
A good agent can find many other ways to make money from your novel. For instance, your agency may be able to negotiate sales of foreign rights or movie options. This is one of the major perks of working with an agent instead of negotiating contracts on your own. They know the proper avenues to earn the most money from your novel.
The bitter truth is that most published writers need a day job, or to do freelance work for magazines, journals, websites, and newspapers to pay the bills. Only about 5% of writers can make a living on their writing alone. J.K. Rowling and Dan Brown-level success stories are rare.
It also would be a wise move to reinvest some of your earnings into additional marketing. While your publishing house may do some publicity for you, authors are expected to work on getting their name out as well. Many authors choose to hire their own publicist to work with the house publicists.
Pushing your first novel is especially wise, because if you do not sell out your advance it can be more difficult to sell your second novel.
However, don't let that stop you from writing your novel and attempting to find an agent or a publishing house. The sense of accomplishment from seeing your work on a bookstore shelf is worth the effort. Happy writing!