Whether you're buying your first kayak, adding to the fleet, or selling one you already own, price is often at the forefront of the decision making process.  What is a 'good' price?  This depends on the following factors:

-Length.  The longer a kayak is, typically the more expensive it will be.
-Materials.  Kayaks become more expensive as you progress from cheap PVC inflatable kayaks, up to plastic kayaks, and on to the premium fiberglass and composite models.
-Features.  Deck rigging, sealed bulkheads, and more comfortable or adjustable seats affect the price of a kayak.  Add on additional dollars for a rudder.
-Age and general condition.  For any previously owned craft this becomes an important factor.

Okay, so what can you expect to pay for a used kayak?  Based on 2010 asking prices on Craigslist, here's a sample of what you can expect for asking prices:

12' Recreational Plastic, or low-end inflatable (Sevylor, Stearns) - $150-300
13' Premium Inflatable (Innova, Aire) - $450-700
14' Recreational/Touring Plastic - $500-700
16' Touring Plastic - 600-900
17+ Sea Kayak, older  (say 6+ years) - $1000-1500
17+ Sea kayak, newer -$1500+

Now, if you are persistent in your search, you may find deals to eclipse the above observations...if you do, don't skimp on other accessories like a comfortable PFD or a lighter weight paddle! Finally, don't forget transport!  A benefit of inflatable kayaks is that you don't need to purchase expensive rack systems and kayak carriers that go with them.