My thoughts on that were, how much extra marketing will I have to do to get that extra 50%, and how many sales would I actually get from my efforts. I read through every page on
http://www.fonerbooks.com and used some of their sales figures. Since I write fiction, I doubt I would have nearly enough sales to make it worthwhile. Plus, if I push people toward amazon or smashwords (for example), that increases my rank on that site and increases the change a casual browser will see my title and click through. That was just my reasoning why I decided not to pursue selling directly on my website.