For Have Gun, Will Play, it's one in ten (on and off Amazon, not including solicited reviews). For other books, it's hard to tell, because I haven't sold a lot and I think most of their reviews came from solicitations.
I'm assuming that HGWP is highly unusual. Part of it is probably because it hasn't sold a lot yet, so it doesn't take much to throw off the total, but I also think it's because it's really hard to get people to read it, and when they do the surprise makes them want to review it.
I suspect, though, that every book is different. Each appeals to a slight different demographic. And your marketing exposes the book to different groups of people. Plus some reviewers are kind of a culture on Amazon, so if you hit a pocket of people who all like to review, and they tell each other about your book, they'll probably all review it.
Camille