I would spent (and have spent years trying putting together a book that was important to me), but I wouldn't spend any time trying to write a best-seller. First, I wouldn't want the stuff that goes with writing a best-seller. Second, I don't really think aiming to write a best seller is the motive or approach I want with my "real" writing (which is offline writing, not the stuff I do online).
The worst thing in the world for me would be to be expected to go out and do book signings or be out speaking in public, pushing the book. I was once invited to be featured on a local cable news "thing" as someone who writes online and supplements her offline work that way. I was told it would most likely get me more traffic online. I had to decline. "Oh the horror of it all!!!" LOL LOL I can't even stand it when I see my articles on the first page of a search. LOL I'm presently in the process of trying to find ways to bury my stuff so that nobody finds it. LOL So, as you can see, a best seller would be a "horror show" for me. :)