Depends on the usual...
Condition, appearance, is it fully functional, any
dodgy switches, sliders, does every section work, what power supply, etc..
can't see any around for sale at the moment but one went for €399 back in 2006 which
might be a rough guide.
The trouble is the Farfisa name and the fact that the
Synthorchestra wasn't very successful. Klaus Shulze used one but that's about it (as far
as I know).
But the vintage keys market is an odd one - to someone (like a
Klaus Schulze nutter), it could be worth £1,000 ... someone else might be reluctant to
give you £50!!!
/ Music Lab Machines