It depends on the post. My blog is primarily about Autism and my son who is on the spectrum. When I post about that I get about 25% of my readers commenting, since a lot of them are moms of kids with special needs.
I've never calculated the exact percentage, as the majority of our site's visitors are actually not bloggers. I think that makes a real difference. On my personal site, which I don't SEO or promote in anyway, gets a handful of comments each week, though. Maybe 20% participation...?