Not as rare as people would like you to believe.
It's probably more hard to find with it's box than the SW Droid Factory and its not exactly a piece of cake to find a complete (or in this case practically complete) Droid Factory. With all the small parts it's no wonder practically ever D.F. auction I've come across has been incomplete. It took me buying two sets just to make one complete one. With that said, the Droid Factory itself isn't a big money maker, in fact in the current vintage market a lot of the playsets/vehicles which were going for big bucks just a year ago aren't selling for too much nowadays. If you go about selling the ESB Droid Factory, you probably will only get bids from those who are serious about collecting items in every form of packaging they came in. At the same time, since the item is opened it won't go for as much as if it was still sealed. Nevertheless, a great find, good luck!