Octopus, Welcome to the net. You'll find this site to be a deep well of expert advise from many great sailors. I sail a 1983 C22 and it's a great little boat. If the one you are looking at is in good shape, has been well cared for, etc., $5500 would be a reasonable price. You can probably even get it for less, given the nature of the used boat market these days.
Does the boat come with a trailer? What is the age/condition of the rigging (standing and running)? Any leaks above and below? Any soft spots on deck? Beware of any deck fittings or port lights that have globs of sealant around the edges... could mean leaking. These things may give you leverage for a lower price. The condition of the keel, keel-cable, keel pivot-pin and hanger assembly can also influence the value of the boat.
I've sailed mine with little ones aboard, dogs, girlscouts, etc. It's a great platform for the family on day sails, and even one or two overnights (although it can get kind of tight after a couple days).
If this boat isn't really cherry, look around some more. There are lots of other similar boats out there, and literally thousands of C22s. Good luck and welcome to the net.