Interesting article from Cringely, where he argues that Google punishes you for lowering your amount per day in AdWords. The experiment went like this:

$0.10 per day for about 15.000 clickthroughs / day
$1 per day for more than 15.000 clickthroughs / day
$0.40 per day for about 1.200 clickthroughs / day

In truth it was a bit more complicated as all tests were done to an identical test site, but it all leads to these numbers.