I think you are missing an important consideration here and that is the production cost of an image.
If it costs $20 dollar to produce the image then I wouldn't be happy until it covered these costs ...
The cost of producing a good series is much higher then the $20 mentioned.
- finding/hiring models that suit the concept
- props
- make up
- clothing
- studio time (own studio, never the less, time spend in studio, on average 8-10 hours per session)
- editing, keywording, uploading
And then, if i have a few sales on first week i might consider myself lucky these days.
But, as mentioned above, it's the overall quality of the portfolio that determines the success. If i upload some pictures that are not really in big demand, but if a designers search for it and discovers my portfolio... that will improve my visibility, hence my sales.
It is hard to determine how many a single photo should earn to be considered successful/cover the cost of producing.
Just my two cents.
Patrick H.
Excample : in the past i did more 3D illustration than photography. I have some series on body building... a few weeks ago i started uploadiing studio pictures taken and with a young body builder... now that lead to a boost in my sales on the 3D body building illustrations.
Simply said, one picture can help/boost another one in sales.