Hi Greenthumb,
1) Google doesn't publish how much you'll earn per ad/niche. But you can assume that sites which target popular keywords such as sex, money, employment, etc. will have higher paying ads, thus meaning you'd earn more per click. However the downside to creating sites in these niches is that there is an awful lot of competition so getting ranked well for these keywords is extremely difficult.
2) Google is ingenious for creating this program. If you think about it, the reason Google is so popular is because it returns relevant results. So this attracts advertisers to advertise on Google - that's how they make their money.
By creating a program like AdSense, they encourage people like us to create content rich sites that only helps make their engine more useful and more popular to keep attracting even more advertisers. That's the whole bottom line - the advertisers. In addition, with us helping them advertise by displaying these ads on our pages, the advertisers are getting more traffic to their ads. So it's a win-win-win for everyone.
Google gets paid by advertisers, advertisers get more traffic to their ads because we are helping them advertise. And because we want to make money we keep creating more content for our sites.
So yes, Google will gladly pay us part of their advertising revenue because we are indirectly helping them attract more advertisers through building useful websites.
Gotta luv Google.
