I understand your question and often think the same thing myself. The most gain in flipping sites is in the middle - sites between $1500 and $4000 as these are fairly low investment and quick turnover, but the problem is in automating this. When you try to manage five medium sized sites, you can end up losing entire days :)

I'm working on a new project to manage outsourced web marketing and ultimately semi automate the process of building traffic and revenue to a site. Flipping is, at the moment, only sustainable if you know how many sites you can personally manage (or even how many people you can manage if others are managing the sites for you).