Using Adwords Bid prices To Detect Strong SEO Keywords

Lately I'm doing a lot of Adwords buying and acct management for smaller local businesses.
I personally believe Adwords, long term, is not the best deal for most local businesses and I tell the clients this.

My strategy is to get them going with ADwords for some quick, but sometimes expensive business leads, and then use the knowledge gained about what keyword phrases are worth building SEO sites for, etc. This way they realize a long term benefit from all the adwords spend.

I really comb thru the most highly bid keywords to see what the competition is wanting to have their ad appear for. . Then I compare those phrases in Googles External Keyword tool to each other. I also use some other free KW tools to get more confirmation.

I assumed the KW's competitors would be bidding on in this particular offline niche (its a service business) would also show themselves to be highly searched terms, BUT that's not proving to be the case.

Instead, they're often very lightly searched if at all, yet commanding some of the biggest prices locally in this niche???

I have some theories, but I wondered if anyone could explain what may be going on? Thanks.
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Bruce