When I started with adsense in July this year I had adsense on all my pages. Started getting low eCPM in October so removed adsense from low-paying pages hoping to increase revenue. It didn't work. I have put adsense on more pages now as I believe while you can't control eCPM you can control the amount of page views you receive by putting adsense on more of your pages. Just my 2 cents (1 click) worth.
Without doubt you're right. I think to make above say $300 a month on a content-based website you need 100 pages of content upwards and at least 1000 visitors a day.
I add adsense to any page that has a decent amout of content on, i DO NOT put adsense on pages like terms of use/copyright/disclaimer/privacy policy pages , that might just be me but i think adsense shouldnt be on them
I realize I'm insane of course... But I have this strong impulse to bid on adwords like privacy policy, terms of service, shipping terms etc. and point the ads to my privacy policy page.
It would be worth if I just made one stressed out web developer laugh seeing a relevant ad. Excellent shipping page Terms of service in finest of print. He clicks and yup! It's a policy page!