Answer: Katharine, thanks for your question. According to your math, you will actually lose money, because you'll need to spend more than just $5,000 on a website. You'll need to create other marketing materials, such as newsletters, speaking demos, etc. I've heard some literary agents say that authors should expect to spend around $10,000 - 15,000 to fully help promote their book. That's the bad news.
The good news is that you can probably get a website done for less than $5,000 if you shop the project around to several designers. Also, your book is really a "loss-leader" to generate more profitable spin-off products, such as speaking engagements, audio and video teaching products, teleseminars, coaching, etc. When I was a full-time author, over half my income came from speaking and spin-off products. You've got to sell a lot of books to make a living...that's why it's important for authors to create multiple revenue streams.
For more information on this topic, check out my article called "Think Outside the Book" at: http://www.startawildfire.com/thinkoutside.html