Here is a pretty good resource to start with: pokemonprice.com by our very own @pierce . It’s a collection of all Pokémon PSA-graded sales, including lots of awesome graphs. But perhaps @pierce could tell you in more details.
PS: The Pokémon Price is also a button in the menu at the top. 
EDIT: I see @gottaketchumall beat me to it. 
Greetz,
Quuador