As a starting point, you can check Mac2Sell, eBay (including completed sales), and Amazon for prices of similar models, to get an idea of what a reasonable price may be. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc. You can also check the Apple refurbished store to see what similar models are selling for.
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I will also add that if there is AppleCare remaining on it, and it expires about a year from now, then I would add roughly ~100 onto what I would pay for it (generally $100 for each year remaining of the total extended warranty period on a used system.)