I think someone mentioned this already, but I am too lazy to give proper credit. This will most likely be like the Lego store in Downtown Disney where there are bins of parts and you make the figure you want, Lego mini figures come in a three pack for 7 or 8 bucks. I would speculate that these droids would sell one at a time. There will be instructions to keep people from jamming parts into the case. (even though that is what is the most fun about the Mr. potato head).
We did the Lego store on our last trip to Disney, and my son loved picking his own figures. I can't imagine what he will do when he sees this.
Chances are though for completests that it will be very expensive to collect all the parts. I would guess two legs, a body, head, and Disney accessory will run about 12 dollars.
I will buy a couple (read as many as my wife will allow) but I can't see getting them all.