The price system that is presented in the MMII is way too linear. Instead I would use the following system.

Price of Warbeast = 25 * HD ^2 gp

This would make the Warbeast Rhinoceros example cost 25 * 9^2 or 2,025 gp.

This scales better, making truly powerful warbeasts pretty expensive. It also has the added benefit of matching the price of the heavy warhorse to the formula.

I would place a giant caveat on this though. This assumes that there is a culture surrounding the beast that engages in breeding and selling the creatures. As a one-off type of creature I would up the price AT LEAST tenfold.