I searched now for a time for an answer on that topic and interestingly didn't found anything related. Not from Appcelerator itself and also not directly from the community - so maybe I'm also completely wrong and there's a reason outside why nobody is asking. ;-)
I'm using Appcelerator Studio with a Indie plan and currently I'm also doing some research on ArrowDB, which is very interesting for my future plans. The documentation for pre-built services or handling with custom objects whatever is - in my perspective - very clear and understandable. The only thing where I'm a bit uncertain is the storage in ArrowDB. In my Indie plan is 5GB included (read that somewhere in the pricing plan overview of Appcelerator).
When I force to use ArrowDB in my app than it could be that the storage is filled over time (sooner or later anyway) - but what happens then? If I focus ArrowDB as a backend solution for my platform I'm running into a big problem at this time, because my customers want to use their services and if the cloud storage is full it could block completely...
Does somebody have some experience with that? Does Appcelerator send a mail before the storage reachs the 5GB limit with some additional pricing elements to extend volume of the storage only? How it's handled?
Looking forward to get an answer on that. Thanks a lot!
Although perhaps there's other users who can share their experience with exceeding limits, this is not really the best place to ask. I suggest you contact Appcelerator sales (http://www.appcelerator.com/contact-us/).