I doubt $50.
Look at the existing streaming services. I think it's safe to say they are generally priced in the $35-$65/month range. However, how many of them are running with the Apple margin built in? If we assume Apple gets about the same (cost) deal as major competitors, then tacks on their margin, I'm thinking $50 would be a challenge for just the streaming service piece alone.
Then, add in the ultimate VOD of "unlimited streaming of all movies & TV shows in the iTunes store." The iTunes store is not full of B-movies & shows like much of Netflix. There's plenty of relatively new and coveted classics in the iTunes store. So I don't see a Netflix pricing add-on for unlimited access to all of that. In other words, I don't see tacking on VOD access to everything in the iTunes store at $10/month or $20/month or maybe even $50/month for VOD alone.
My Guess: For a comparable streaming service with the Apple margin, I'd guess $50 might be possible... but probably >$50 or maybe tiers with $50 as a base tier but more toward $70+ to get most of the more popular channels or programming. For unlimited iTunes VOD, I'd (wild) guess a monthly fee at maybe $70. Or again, tiers of access where maybe another $50 could be in there for something less than "unlimited."
Basically, the existing model revolves around monthly revenues of (I estimate) about $125/month per household. That's about $75 for traditional cable/satt on average + about $50 via the OPM subsidy for commercials. I think any good replacement model to be pitched to all of the players involved probably has to START at persisting that level of revenue. Else, which player is going to take the big cut to let Apple pile in on top and get theirs?
I'll hope right with you that Apple can roll out a competitive streaming service, maybe with live programming too and unlimited VOD access everything in the iTunes store for only $50/month but I just don't see it... unless Apple is going to subsidize it to hit that target (which I don't see happening either).