If you are writing because you want to make a packet of money, then retire famous and move into a big house with a pool, chances are you haven't read the fine print.
Even if you sell 50,000 books, that's chicken feed as far as profit goes.
But don't kid yourself. The statement, 'I just have to get my book published, but I am only really doing this for myself,' is a contradiction. If you were only doing this for yourself, there would be no need to publish. You've already read it.
So why are you doing this?
I am doing this because I can't stop. Because once I begin a story, the story itself whisks me along in directions I did not expect, engrossing me in what could be. And I have to keep writing because I simply must know what happens in the end. For me, it's very much like reading an exciting adventure, if I don't keep writing, I will never know the outcome. And every time I write another book, another great adventure colours my life.
So if you have a tale to tell, if you need to get a message out there, if you simply must find out what happens at the end of your tale, and if you want to have an impact on others, then write your heart out.