I think this is a question that each writer and author needs to consider. I don't mean to suggest that you should base your idea of success on the sales of a book, but rather, how much money can you make by publishing your book? If you write a blog, and then plan on improving your writing you will earn some income, but you will have to publish a book to know if you can make a lot of money.
The most important thing to know is that you have to improve your writing if you want to have readers buy your book. This is a good reason to go to writing classes and build some online articles, so that your mistakes are caught early on. It is about making money but it is also about branding and being professional in all areas of writing.
Before I go on a personal rant, I would like to clarify something: I am using my own book as an example, and I am adding the links so you can see the numbers.
I think that if you sit down and ask yourself a question like "How much money can you make publishing a book?", you might be in for a big surprise, this is most true if you did not bother to spend time with an editor or have a plan to improve your writing. Even if you publish traditionally, you cannot simply sit back and wish for your book to sell. The work is not easy, it means that you will have to market your book and build a following, and spend money to market your book some more.
I don't know of an author who hasn't made some money publishing a book, but it's not quite the amount of money that will let a person quit her day job-- this is most true if you self-publish your book. My first book, In Search of the Lost Ones, is doing fairly well; however, if you go and look at the numbers on amazon.co.uk or amazon.com and either at the Kindle edition or the paperback edition, you won't find much to get excited about.
Since both of the Kindle editions were released, and they were free to download, I've sold about 50 copies of the book on amazon.com. I did put it on Kindle Select and then there were a number of days when the e-book was available for free. Yes, I wrote "free." (Bear in mind, I probably sold over 100 copies during that free promotional period. I am certainly not going to complain about close to $100 -- but I won't say that that is a lot of money! )
On Amazon.com I've made roughly $100 whereas, on Amazon.co.uk I count about 25 e-books sold and about 50 free e-books, so I have made closer to $50. That is not a lot if you factor in how much time it took to write and edit this book. That's not much money you can argue.
With these numbers in mind, I can say that at least I have made money publishing my book, but I am certain that others have made more... and less. That is the nature of the publishing industry. It is not for the writer who thinks that it's quick and easy, and that marketing will just happen.
I am not saying that paperback sales are as high -- for example since I live in Canada, my amazon.ca sales are much higher than those on amazon.com. I have sold about 250 copies to readers in Canada from amazon.com, and I have sold about 200 copies on amazon.ca in a shorter period of time, but In Search of the Lost Ones doesn't have as much competition here as it does in the United States. My book is usually in the top 100 in the category History, Europe, Eastern.
I am happy to say that a few times it's even been in the top 100 of the History, Europe section, but that was shortly after it was published. It will take a lot of marketing to continue earning high ranks like that.
It is true you can make money publishing your book in a more traditional fashion, and the same holds true for self-published books, but maybe the part most people forget is "how much money can I earn."
I would like to make more than $10,000 in profit when I think of my current book and future published works, but I also know how much work I need to put in to achieve that. Yes, you can make a lot of money, but it's not an easy road, and I don't think it's a way that you can make money very quickly. It is a snowball effect.
I am sure I can make money - I already have - but 'how much?' is a very personal question for each author, and it is one that can only be answered by that author of the book. So, how much money can you make publishing a book?
Become a Follower of Living a Life of Writing, and read it in paperback In Search of The Lost Ones: The German Soldiers of Transylvania in the Second World War and Their Stories and on Kindle In Search of The Lost Ones: The German Soldiers of Transylvania in the Second World War and Their Stories and also at Amazon.co.uk and Amazon.de