i'm putting mine at $3-$4 .. but not sure what is the best. I figure though, if they want the image they will be willing to pay $3. However on the other hand, if they might shop around to other sites and just buy it from dreamstime.
However, that being said, if they are buying images from feature pics, is is probably because they don't know about dreamstime. Why would someone buy from feature pics with a limited selection when they could shop at dreamstime or another site and get the same selection possibly cheaper.