I'd be quite curious how the AdSense revenue has dropped off since 2006, as the platform has evolved a lot since then, not to mention people have had a long time to get used text-based contextual advertising.
It was a lesson I'll always remember - if you're going to build an ad-revenue-based site, then do so in a field where there are major players throwing around big money on adsense or similar. You get to effectively monetize their users by providing an advertising channel.
It is not field based. Every company has their own bidding budget. Take any field, you will be able to find clients who are ready to pay higher CPC than avg because they have money
Credit Cards and Insurance will always have higher CPC than florists. That's inherent, and that's what I mean by choosing the field carefully.