How do I find expensive keywords in AdSense?

You can find expensive keywords easily with Google Keyword Planner. But what you need is efficient keywords which are of low competition (not in keyword planner, but in google). Try to find any local keyword. Local keywords have very few competitors and good amount of searches. It won't be a huge CPC but will be more effective. Search your state in keyword planner and try to find out a keyword from the suggestions that shows up in keyword planner