A first-timer can sell 5K in hardcover and another 20K in paperback. Really, that's not outside the realm of belief at all.
Depending on your genre, and depending on your market, and depending on the way the dice fall.
The advance is figured on how much the publisher thinks the total royalties of the book is going to be. Usually they're right. Let's see: $25 hardcover, 15%, 5K copies.... $18,750. $7.00 paperback, 8% royalties, 20,000 copies, another $11,200. $29,950 advance (call it $30K). Divided into three payments of $10K each, 15% to your agent, 20% to Uncle Sam...
What's a writer to do?
Answer: Two books per year.
If anyone knew how to make a book a best-seller, then every book would be a best-seller. And you're relying on the public. They're fickle, and unpredictable. So, have fun with the writing. That's the thing that is under your control.