I think that your conversion rates with Yahoo answers will depend more so upon the niche than anything. If you are promoting services to a more serious crowd, then your results will be better. So it depends on your product as well. If you are targeting "buyers" then you'll probably get a better response than if you were to go after people looking for free stuff. You also should be sure that you are giving quality answers. While just posting a link might give one or two visitors, a high quality answer should give you some decent traffic. And of course, the more traffic you get, the more conversions you stand to gain.
I am getting at least 5-10 visitors daily from Yahoo answers. But all depends on your niche. I have tried on make money online niche. It didn't work for me. But for pet niche, I am so successful with Yahoo! answers. Hope this will help you to choose...
I think with Yahoo Answers, people are going there for "answers" not to make a purchase. So it may bring traffic to your site, but not many conversions. Which seems to be the consensus here.