If this is a real market for this kind of paid service, I believe that sets up the correct financial incentives. It also solves for scaling issues, troll issues, and financial sustainability of the company.
If I can't validate the market via the join.app.net campaign, then we can still say that we tried, and won't take anyone's money/let anyone down.
Happy to meet in person
From a user standpoint: One has no reason to join. Adoption occurs when the benefits of joining are greater than cost of joining. You inflated the cost of signups from 30 seconds to $50. For people considered wealthy, that's still over 100x the cost. Having no ads is hardly 100x the return, especially when you still have a massive chicken/egg problem to solve. It's like trying to make Google+ overtake Facebook. It's hard to begin with - it's impossible if it didn't have the resources of Google and wasn't free.
From a developer standpoint: The service is useless without far more users than your asking for. The data won't be compelling with 10k, especially considering the personality that App.net currently appeals to is so monotonous. As much as I like to hear NoSQL vs SQL arguments, it's pretty fucking banal when you compare that to someone on the other side of the world tweeting about war or a natural disaster. Twitter is as much an outlet for media as it is an inlet and the friction of your monetization strategy greatly hinders who can tell their stories.
Sustainability doesn't mean viability. You'll make money no doubt but that has really no correlation to the quality of the service, which, unless it's free, pretty much fucks everyone but you.
Twitter will be real time in due time. In the meantime, I urge you to stop striving to be Twitter and instead more like maybe Meteor. Making a good real-time framework seems to stick to the idea's roots while filling a need that is actually needed. However the project develops, I truly wish you the best :)
The barriers to entry as a messaging infrastructure service play are low. Even if your platform is reliable and stable, you will be quickly commoditized to competing on price which is not a sustainable business model.