One way to do this is to export your top 20 organic keywords sorted by visits from Google Analytics. Then take that list and copy it into the AdWords Keyword Tool to see what the local search volume is for each of those keywords. Then you can easily find the percent of the Local Monthly Search Volume that your keywords are getting.

One step further is to see what would happen to revenue if you increased those keyword visits by something like 5%. Using the Per Visit Value from Google Analytics you can take the difference between the amount of visits you are currently getting and how many you would get if visits were 5% higher and then multiply that by the Per Visit Value to see what the gains in total revenue would be.

The prospect of taking a keyword from less than 1% of total Local Search Volume and getting just a few percentage points more seems very doable especially when the increased revenue looks so high.