Few facts:
Utility will dictate price MUCH more than rarity, though rarity does still contribute a lot. The cheapest lvl x's (almost always Dialga, much to my discomfort) still go for $10, while the most valueable shiny's tend to top at $5 (Duskull). Secret rares almost always garentee value. The cheapest, charmander, still hovers well in the $15-20 dollar range.
Still, the most valuable card in the game definately seems to be Leafeon X, which consistantly sells at my local for a good $50, even when Time Space Distortion and Charizard sit in the exact same case for $30.