The fluctuation there depends on things like storage space and 3G/LTE capabilities. Adding those features adds cost, naturally.
Considering that the devices start retailing at $499 and go all the way up to $929, it's only good news for Apple.
The iPad Air's display is more expensive than previous generations, but the components still end up totaling $42 less than the base model third-generation iPad because there's not much else new on the inside.
Yes, there's a new Apple chip, the A7, but at an estimated cost to Apple of $18 (Samsung still manufacturers it for Apple under contract), it costs five dollars less per unit than the A5 processor did 18 months ago. Dialog Semiconductor produces a power management chip that goes with it, though this one is bigger than one seen in the iPhone 5S.