While I was developing River of News I looked around for any discussion I could find where developers revealed their sales or the relation of ratings to sales. I found several sources independently say that about 1 in 30 buyers of a paid app will rate it in the store. I now have two apps in the store and that ratio has held up remarkably well.
I'm sure there are exceptions and my information is far from comprehensive, but I have enough evidence to believe that multiplying the number of ratings in the U.S. App Store by 30 will give you a ballpark estimate of the sales of an app.
I'm curious to hear from other developers if this ratio is accurate for their products.