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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beetwen 500-550 pounds tops would be a good asking price. Good care of the machine, remaining AppleCare and no cosmetic/functional issues can help in the price.
And dont forget to account shipping costs/fees if selling through an online site.