Amazon Prime LTV is on the order of thousands of dollars per member. If they can convert new customers to Prime for ~$100 that is pretty cheap. Of course, media is a hit driven business, so they would have to blend the cost of all the shows that don't do as well as their headliners.

I personally don't like Prime shows other than Man in the High Castle. And they are increasingly pushing down other New Releases so you only see Prime shows above the fold on TV devices (Roku etc). But I understand they are playing the long game here and quality will probably improve. I used to dislike most of Netflix original shows, but now theres a steady lineup that I could watch regularly every week.