As a starting point, you can check Mac2Sell, eBay (including completed sales), or 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.
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everymac.com, the base model (4 gb RAM, 250 GB HDD) has a retail price of $850-$1100.
Personally I think you can estimate a better price taking in account a depreciation factor of 0.25/year (if mild use) or 0.33/year (if heavy use) over the price you pay for the computer. Also factor cosmetic condition and Apple Care, if applies.