One could read it both ways As is (the more the better) or the reverse if (as is) then I will be lost in an ocean of sameness!
The more the better, very true. Since different tools uses different engines you are expected to see some big differences. Wordtracker.com is one of the oldest with a little more accurate result and goodkeywords.com uses overture, teoma and altavista. The real count won't matter as the top keyphrases with high relevancy will be of high importance for your business anyway.
ROI do you mean return on investment if so how could you give a Dollar value to the result or ROI has another signification, alien to me!
As I said this was written for last era of SEO where you had to invest for each KeyPhrase for ranking. People use to hire link exchange guys and pay between $2 to $10 per link.
Now say for a big keyphrase you might need 10000 links and that makes it $50,000 investment. But if you rank good you can easily get 10,000 odd visitor every month with a conversation ratio of 25%. That makes it 2500 customer every month. Then the profit game starts. Now getting 10000 links will take much time, say one year.
On the other hand there is a small keyword which needs just 100 links giving you 20 converstion every month with an investment of $500 and which can be achieved in a month or two.
In other words these weird calculations will help you decide an optimize investment plan. You can calculate various ROIs. I advice to concentrate on smaller ones first. 70% investment for small keywords and 30% for bigger one every month. After one year you can redefine the strategy based on the progress.