Yahoo has a tool that allows you to
enter a keyword phrase and see all the searched variations on that phrase for the last full month. The results tell you the number of searches for each phrase variation. Use it to locate opportunities in phrases with smaller search volumes that might be less expensive to bid for, as well as those that might be overlooked by the competition.
In the image you'll see that, once you've entered a keyword phrase, the list of variations and search quantities are displayed. It's possible to find niche phrases for your expertise, such as commercial or investment. You can also target the lower volume search phrases, as much of your competition might not be bidding on those or bidding low thinking they're not that productive.
Flash video of this tool in use