The web app I run on AppEngine 24/7 is a low volume site so I never appreciated how much the daily free quota gets you until last summer when I took one of the Google Buzz firehose reading demos and modified it to extract what I wanted from the Buzz firehose (large volume of Buzz, Twitter, etc. social messages). I just used the daily free quota and my temporary app would run for 5 or 6 hours a day before hitting the quota. Lots of bandwidth and CPU use.
That said, I never mind my monthly Amazon AWS bill because its a good value (and, for a long time it was free because of two grants they gave me).
I also would like a plan from Google where I gave them some money every month; this may happen when version 1.4 is released because I am trying to decide between AWS and AppEngine for a new project. I am curious what having three instances always spun up will cost.