The price of an organ depends on the type of organ. An electric or digital organ will be the most affordable type of organ and can be found for as little as $50 depending on the age and condition. Meanwhile, a full pipe organ can be found for $30,000 on the low end to close to a million dollars.
You can also find Hammond-type organs for relatively low prices for the same reason you can find pianos for free -- sometimes folks just don’t want to move them and will happily give them away for free to someone who will.