If they raised the minimum bid, they would have to raise it for everyone. And I am not so sure that doing this would cost Google a lot of money. I keep reading in the forums all the time where affiliates are complaining about their keywords constantly getting slowed or stopped. If all of the ads are going to different domains, their users would probably click on more of the ads, so even if the bid was less maybe the clickthru rate on the ads would be much better which might mean more money for Google. I am an affiliate but send the traffic to my own site, and I rarely have any problems with my keywords.