CardCastle now uses MTGStocks.com for our card prices.
MTGStocks amalgamates card prices from various places, including TCGPlayer and MagicCardMarket, to give accurate prices based on what people are actually paying for the card. They also protect against false price spikes when cards are listed for unrealistic prices and don't sell - for example, if Jeskai Ascendency is worth .25 cents but three are listed for $420 each, the MTGStocks price for Jeskai Ascendency won't be affected.
We are currently working on an issue where certain foil cards that are printed in some supplementary products don't have a price listed in MTGStocks. These cards will only show the non-foil prices. We are aware of this error and are working to solve it as soon as possible.
Click here if you would like to learn more about the MTGStocks pricing system.