If it's simply a larger version of the same ideals (eg. Pro is to Air as Air is to mini), then it would be logical to assume that you get all the same models at $100 more than the equivalent Air, meaning it would start at $599.
However, if the Pro is more than just a larger version of the same iPad concept (eg. some kind of 11" MacBook Air alternative, greater functionality than today's iPads), then I would expect it to start at say $699 or $799, positioning it above the regular iPads without causing higher capacity/cellular models to break $1000.