Judging from the fact that most recently approved themes sold below 50, and be considered lucky to sell a 100 Copies.
Which theme was that?
If it was approved only recently, 50 sales can be called fair.
new releases help my old items to gain more sales.
Everyone use some framework, so production of new theme is fast and cheap. 50 - 100 sales are still ok 
I think there are 2 tactics
- short term: fast and simple, low sale
- long term: big and complex, big sale, but maybe slow
