It's impossible to say that, because you can't tell 100% whether your article will stay on top spot for specific keyword or not. If it doesn't, then you might make few sales from the article, but nothing long-term. However, if it does, then your earnings are limitless, unless someone outrank you.
It is also impossible to say if you have several sources linking ti the landing page/lens/blog... Let's say I have a squidoo lens which has 5 articles linking to it, and it's also ranked high in search engines... I really can't tell where the sales come from - SE traffic or Article traffic... However I could tell you that most of my articles get 50% CTR...
Agree with everyone above but if you want a nice little average I have made average $15 an article, but some have made 0 some have made lots, some i have had thousands of views others none. And I have counted the first hundred or so articles in there in which conversion rates were terrible
Some of my articles bring in lots of sales, some get no traffic (or little). Really depends on your long tail keyword (how good it is, how the traffic for it is) and how you SEO it (get it to a top spot on google and keep it there).
as far as i'm aware ezine hasn't directly brought me any sales ... it has brought me traffic ... but on a page that normally brought me about a 1 in 30ish sales ratio, i got hundreds of visits but no buys ... article traffic not targeted enough? may have been so ...
oh really grt,how many clicks are you getting for this article,which is converting into sales, and are your articles on top spot of google.
Per article I get about 30 clicks... And they get pretty good ranking - always 1st or 2nd page in google SERPs. I'm writing one article per day... I don't really know which articles exactly convert but I'd guess every single article of mine converts because I always get one sale right after the article gets published.