There is no easy answer. There are no appraisals. There is no pent up demand for Farfisa organs (that I know about) anywhere. Not the answer you want to hear, but sadly it's true.
Here's a brief history of Farfisa organs in the US and how you should judge trying to sell (or gasp) give one away:
Wood console organs were something that people had in their homes. If they were musicians and couldn't afford a piano, they had an organ. Television came along and ruined that. While these organs are nice and rich with sound, you will find hundreds of them in your local craigslist listing everyday. Most of them are for free (if you and your buddy will come over and take it away). It doesn't matter if they are made by Farfisa, Conn, Wurlitzer, etc... They have little to no value.
Reed organs like the Royal Artist Piano and the Portorgan are somewhat portable, but need electricity, and are closely related to Farfisa's original accordions. Due to the fact that the fan is needed inside the organ to make it work, there isn't too much of a demand for these from musicians. They are just too noisy to mic up. They have little to no value.
Farfisa Compact organs came along in the last sixties for the touring musician. They were a hell of a lot easier to carry than a Hammond! The later VIP and Professional Series organs were more evolved (but not as cool sound-wise if you ask me), but all of them were replaced by digital keyboards that cost a lot less, were a lot lighter, sounded pretty good, and had every possible sound a musician could ever want.
With that being said, after the Compacts, VIPs, and Pros disappeared from the stages, and ended up in pawn shops and used music stores, there came a sort of resurgence. What was passe to one group of musicians, became welcome in the punk and alternative music scenes. Note that in these scenes, it is much cooler to have bought a piece of music gear for $20 (like my first Farfisa) than it is to have paid a lot for one.
If you own a Compact, VIP, or Professional Series organ, a Farfisa Amplifier, or any of the Farfisa pedals, you should research pricing of what they are going for on ebay and craigslist. That's the best advice I can give on how much your Farfisa organ is worth. That and some wisdom passed on from years ago...
It's worth what you paid for it.
-or-
It's worth what you got someone to pay for it.
Good Luck,
Jim