At NonSportRealm.com we use a variety of sources.

• Non-Sport Update
• eBay
• Historical Price Guides
• Manufacturer websites (i.e. if they are still selling for cheaper)

Ultimately pricing is something always in flux.  With there being hundreds of thousands of cards in the database it is a never ending battle.  We have a forum topic called "Price Check on Aisle X Please" where you can request the Mod's to investigate pricing that seems out of place from what you are seeing.