They will update the iPad Air or 9.7 inch model because it's the more mainstream version of the 2. A 12 inch is quite huge and I would think a bit large for travel. Plus price wise $499 for the basic model is quite a lot of money for some people and then increasing it to $599 at least for the pro would lose sales overall.
Its not $599 though is it. Its sell old iPad put that towards the cost. Apple knows this.