CardCastle has two options for pricing data - TCGPlayer Market Price and Card Kingdom prices.
TCGPlayer Market Price shows a good idea of the current 'true value' of a card by drawing from the thousands of transactions each day from just as many individual sellers. This finds a good average and gives a solid representation of the card's value as decided by what value has been bought and sold. We update prices twice a day from a third party provider.
Click here if you would like to learn more about the TCGPlayer Market Price.
Card Kingdom prices are useful for getting an idea of the steady price of cards. Because Card Kingdom decides their prices themselves, their data isn't subject to change as frequently as TCGplayer's.
NOTE: Cards from some supplementary products (eg. Commander 2017) are missing. We are aware of this issue and are working to solve it as soon as possible.