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.
Don't check just once source, but get an average from all sites for a more accurate idea. Some sites may be higher/lower than is realistic. Then adjust the price to account for condition, configuration, remaining AppleCare coverage (if any), etc. If the model is recent enough, you can also check the Apple refurbished store to see what similar models are selling for.