Price that low would not make sense to them. generously assuming 4% conversion to paid accounts (DropBox had 4% at some point) from their 320M MAU, they would make just 15% of their current revenues with your model.
I still think that their best bet is to work on their ad engine. Two of the 4 biggest tech firms, GOOG and APPL, make insane revenue on ads.
I assume you meant FB? Without checking numbers, FB average revenue per user is probably triple that of Twitter. Because of FB's scale, scope, etc, I can't see Twitter getting too close to FBs numbers. But yeah that still means Twitter should potentially be able to double its revenue.