Unfortunately, I seriously doubt you'll find an official answer.
My answer is based vaguely on the pricing rules for enhancement bonuses and several other things, which are usually bonus squared times a constant. For example, weapons are bonus squared times 2000.
With my formula, if you double the creature's HD, you multiply its original price by 4, i.e. 2 squared. If you triple the creature's HD, you multiply its original price by 9, which of course is 3 squared. Coincidentally, that's the same thing that happens when you double or triple an enhancement bonus or other exponentially-growing variable. So... not official, but based on official precedent.
As for the training costs, my reasoning for that is that whether it's a superior specimen or is older/has been trained longer, either the markup or the cost of training should increase consummately. It's simple enough to remove that, though, if such is your preference. Just use 7,000 as the base price and add the 1,500 training fee to the final cost. In this case, the final cost ends up being around 15,800.