Want to spend $55 per click on Google AdWords? Get into the insurance business

As any advertiser knows, Google AdWords are not all the same. Buying up a keyword category like "software" will run you a lot more than "furniture." Even in online advertising, costs are largely determined by supply and demand.

WordStream decided to figure out the most expensive keyword categories in AdWords and just how much they cost per click. Because of the platform's auction-based pricing system, a click can cost anywhere from a penny to a bajillion dollars and a bag of chips, which makes it interesting to observe how high a wide open market will drive some keywords.

At the top of the list is insurance. Accounting for a staggering 24 percent of all keywords, the category "Insurance" commands an insane $55 per click. Read that again: $55. Per. Click.

In second is "Loans," which commands 13 percent of keywords in its category and costs a whopping $44 per click. "Mortgage" rounds out the top three with 9 percent and $47. "Attorney," in fourth, also costs a wallet-devouring $47.

Also in the top 10 are Credit, Lawyer, Donate, Degree, Hosting, and Claim.

The top 20 categories combine to encompass roughly three-quarters of all keywords.