SpyFu is an excellent tool to find out the average CPC (Cost Per Click) of the keyword that you feed it. On each page resides a graph that displays backdated data - allowing you to see whether or not the keyword, that you have selected, has any patterns.
The one thing that I warn you not to get mixed up about is... The price range displayed is not the amount that you will earn in AdSense, it's the amount that you would need to pay if you were to start a campaign over at AdWords.
SpyFu also allows you to research certain AdWords campaign starters, allowing you to find out approximately how much they spend / how many clicks they get on their campaigns daily.