How much is one clickthrough to your web site worth? If you’re a legal firm specializing in bringing mesothelioma cases to court, it turns out it’s worth a lot.

In fact, the search term “mesothelioma” was the highest-priced keyword in the U.S. during the third quarter of 2009. That’s according to a recently-released AdGooroo Search Engine Advertising report.

Just how expensive? For Google’s AdWords program, the highest price paid for a #1 ranking on that search term was a whopping $99.44 per click. (Over at Yahoo, the high figure for this paid search term was a little less rich: $60.68 per click.)

One wonders how many times the advertisers have actually had to pay out this king’s ransom. Whether it’s for a few or many clicks, it’s clear that some legal firms recognize a highly lucrative revenue opportunity in filing mesothelioma lawsuits related to asbestos and lung cancer.

Interestingly, the highest paid search term in Bing’s paid search program isn’t “mesothelioma,” but rather “auto insurance comparison.” At $55.20 per click, the dollars aren’t as high, but it would seem like the potential payoff isn’t nearly as great, either. After all, there’s a pretty big difference between a multi-million dollar legal verdict and the value of an automotive insurance policy.

But beyond the eyebrow-raising stats of these extreme examples, the larger issue is how much more costly search advertising has become in recent times. A few short years ago, it was common to talk about search terms costing an advertiser 50 cents or $1.00 per click. Now, those same terms are far more likely to cost $2.50 or more.

Google, being the 500-pound gorilla in search engine marketing (SEM), has certainly contributed to the price inflation. That’s one reason why many are rooting for alternative search options like Bing to succeed (dream on).

More fundamental to the increase in keyword click pricing is the realization that advertising to people at the precise time they’re searching for particular goods and services is a far more effective way to engage customers and prospects and drive actual sales.

And that’s even more the case compared to trying to get their attention or otherwise “intrude” on them when they’re online for other purposes. The abysmal clickthrough rates experienced for banner advertising bear this out.

But paying $100 per clickthrough? That does seem excessive – even for ambulance-chasing lawyers!