My friend and I are writing a fly fishing book about the renowned and unknown rivers in Oregon. There is a decent audience from this book and we have had two publishers say they might be interested in publishing this book. This is the first time we have written a book and we don't know much about this process, so the question is this: For a first-time writer writing a fly fishing book (good audience though), how much should I expect for my advance? $500? $5,000? I have no idea.
Unfortunately, there isn't an exact answer. It depends on so many things: your agent, your publisher, your book, etc. Some people are getting $500 advances. Others get seven digit advances. It depends a lot of factors and there is no one answer.
Also, just so you know, for non-fiction, you don't sell your book. You sell your book proposal. You'll approach agents/publishers with a proposal for what your book will be, and usually, if they give you an advance, then you write the book.
For good suggestions on book proposals and anything/everything related to publishing, check out writersdigest.com and writersmarket.com or go to a bookstore with a non-fiction section. Barnes & Noble has tons of books on these topics (most published by Writers Digest).