for some reason certain cards are much more in demand, usually anime cards.
Some cards are more expensive because not many people have a game. Other than that card prices fluctuates over time, it get price drop if a game is in a bundle, after some time it rises a little, and then there is price increase of cards at start of a major sale (summer/winter).
edited: bundlestars havein F.E.A.R bundle, if anybody missed it before
http://www.bundlestars.com/all-bundles/fear-bundle/
I might be hallucinating but I think it was cheaper on steam itself, where I bought it.