The Display Planner tool replaced the YouTube Keyword Tool in 2014, and can be accessed from the Tools section of your Google Ads account. Although it's a little more involved than entering a keyword into the YouTube Keyword Tool, the Display Planner offers some great functionality for making the most of Display campaigns, including keyword-driven video ad campaigns on YouTube. The Display Planner also offers precious insights into the demographics and interests of your audiences, allowing you to create highly tailored Display campaigns.
This is a rather crude metric because it presumes one can monetize all the traffic they receive AND one can generate as much profit per visitor as Google does. Anyone who could do both of those would likely displace Google as the first consumer destination in their market (like how many people in the United States start ecommerce searches on Amazon.com rather than Google.com).
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input. In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).