Why such a dramatic jump in price from 2GB to 4GB?

Who knows, maybe because it is registered ram? and DDR3, also if you haven;t noticed, 12 GB Kit that have 3 4GB sticks are upwards of 1000$ last I checked. While you can get 6 2GB sticks for much less.

Mauybe it just costs that much more to put 4GB onto 1 stick and make it work.

Because the 4GB sticks are much harder to make than 2GB. I don't know exactly why, but that's the way it is. Also the fact that it's ECC compounds this.

Harder to make and if you could get 2 - 2GB or 1 - 4GB sticks for the same price what would you buy? The higher GB stick right? They know this so price is high.

Maybe it's different for the 4gb sticks, due to the nature of the stick itself, but ECC was kind of made irrelevant on the 1366 architecture.