WordStream, a maker of an PPC management platform, conducted a study on top keywords and categories that Google makes money from. They analyzed what keywords demand the highest costs per click and are most competitive in terms of high search volume. Here is what they found:

:

The 20 most expensive keyword categories with the highest search volume and highest costs per click, thereby netting Google the most money, are:

1. Insurance (example keywords in this category include “buy car insurance online” and “auto insurance price quotes”)
2. Loans (example keywords include “consolidate graduate student loans” and “cheapest homeowner loans”)
3. Mortgage (example keywords include “refinanced second mortgages” and “remortgage with bad credit”)
4. Attorney (example keywords include “personal injury attorney” and “dui defense attorney”)
5. Credit (example keywords include “home equity line of credit” and “bad credit home buyer”)
6. Lawyer (“personal  injury lawyer,” “criminal defense lawyer)
7. Donate (“car donation centers,” “donating a used car”)
8. Degree (“criminal justice degrees online,” “psychology bachelors degree online”)
9. Hosting (“hosting ms exchange,” “managed web hosting solution”)
10. Claim (“personal injury claim,” “accident claims no win no fee”)
11. Conference Call (“best conference call service,” “conference calls toll free”)
12. Trading (“cheap online trading,” “stock trades online”)
13. Software (“crm software programs,” “help desk software cheap”)
14. Recovery (“raid server data recovery,” “hard drive recovery laptop”)
15. Transfer (“zero apr balance transfer,” “credit card balance transfer zero interest”)
16. Gas/Electricity (“business electricity price comparison,” “switch gas and electricity suppliers”)
17. Classes (“criminal justice online classes,” “online classes business administration”)
18. Rehab (“alcohol rehab centers,” “crack rehab centers”)
19. Treatment (“mesothelioma treatment options,” “drug treatment centers”)
20. Cord Blood (“cordblood bank,” “store umbilical cord blood”)

Source: http://www.wordstream.com/blog/ws/2011/07/18/most-expensive-google-adwords-keywords