Just found a really neat tool that will show you how much you could be making for a particular position in Google for a particular keyword. This free tool will give a pretty close to real world estimate.
The Click Distribution Tool at BestRank should be very useful to determine how much you could be making for a particular position in Google. The link is to a video that shows you how it works. You really should watch this 6 min video because this is a pretty sophisticated tool.
This tool can also show you if a particular keyword phrase is worth ranking for or not. It provides you a good idea of what the position will be worth. Like a keyword tool there is some variation. If you have real world data from logs or an adwords account you can be a little more precise. If you are planning to focus your effort on a certain keyword or phrase it may be worth your time to see how valuable it could be.

