That is impossible to answer as there are so many variables.
What you need to do to make it worthwhile is choose a genre to write about that is in big or at least reasonable demand, find the best tags/keywords to list it against, put the book in the right categories and make sure the content is of good quality.
Then you have to get the title and description right, price it competitively and promote it in every way you can think of. Including through qualified lists, press releases, blogs, Twitter and the list goes on.
If you are lucky it will go viral, but at least if you have done all the above you should at least get a reasonable return on the time and effort you put in if it doesn't.