An official and legal version of this wouldn't be $50/month, it would be more like $200/month. It includes movies from every major company, and films while they are still in the cinema.
What makes you think movies should have a multiple of 10 instead of 1 for a monthly subscription fee?
I'll attempt an answer, the cost should be whatever the market will bare. The market could likely boom at $50/mth, but at $200 it would be niche. The industry seems to even be coalescing around $30/mth (my guess) through consumers paying for multiple services.