I use Yahoo's total results as a marker. That's the big number they display to the right of your search (for example, "1-10 of about 176,000,000 for keyword x")
Then use the formula demand/supply x 1000 for the formula to determine your profitability. So for example, if Overture gave you the following results for a keyword demand...
blue widgets = 3,000
red widgets= 7,500
orange widgets = 2,000
Go to
www.yahoo.com and search for the 3 phrases individually.
Blue widgets returns 4,480,000 results
Red widgets returns 7,340,000 results
Orange widgets returns 1,790,000 results
So using the formula Overture demand/ Yahoo supply x 1000
The demand for the three keywords are as follows...
blue widgets - .700 (rounded to the nearest thousandth)
red widgets - 1.022
orange widgets - 1.117
So orange widgets would be your best bet if you narrowed it down to those three keywords because it has the highest profit ratio.
There may be other ways of doing it but that's what I use.