There's no straight answer to this question. It really depends on whether you want to rip off a desperate musician or give it to them for a fair price. At one time the Rhodes was so despised that you could get one for $50, or even for free. These days a fair price is more like $500 to $750, depending on the condition of the piano. For $500 you should be getting a Rhodes where all of the keys play, and if it's a Suitcase, the amp should be included and functional. A piano in "living room" condition (i.e. one that plays well and has no visible road damage) may be worth $1000 or more. Prices are generally the same in Euros.
Things to consider:
* If it is a Stage model, does it have the legs, sustain pedal and push rod? These parts are commonly missing from "well-loved" Rhodes pianos and are somewhat expensive to replace.
* If it is a Suitcase model, does it have the original amplifier, and does the amp still work? Does the preamp work? And most importantly, does it have the 4- or 5-pin cable that connects the piano to the amp? That strange cable is the most important part of the deal.
* Has it been used on the road, or is it a living room piano? Pianos that have been toured tend to be in need of some loving care, and they will probably need replacement parts. Some parts may be hard to find; others may be expensive to replace. Make sure you don't have to buy an entire piano's worth of parts to salvage it!
* Do any spare parts or replacement tines come with it? If so, fresh tines or other parts could really sweeten the deal.
Other things that can make a Rhodes more valuable are third-party enhancements like Dyno-My-Piano modifications, an aftermarket preamp, or a MIDI retrofit. These uncommon features can add hundreds of dollars to the piano's asking price.