I realize 2GB would use more power than 1GB. However, I've actually looked at DRAM specs for desktop memory before and the increase in power consumption usually isn't 2x. It was like 50% or something. Granted on a device running on battery, every milliwatt counts.
And yes, Apple has never announced the amount of RAM for iOS devices but it's something that manifests after a couple of iOS upgrades. As I mentioned there's no hard need for a RAM upgrade now. Performance in iOS 7 is pretty good for the most part so there's no incentive for Apple to increase it. However, 2 or 3 iOS generations down, the 1GB RAM on the iPad would likely limit performance and start making the device feel sluggish so users are more inclined to upgrade.