not sure if it is reasonable... but thats the way it goes.
It depends on the app how much the mongrels use. My current app is about 81mb at startup. Then average about 100mb. There are a few memory intensive calls that bump it up towards 200mb. My only problem with that is that ruby does then not release the memory. Thus I have "god" restart mongrels that are above 180mb for 10 minutes. I'm working the boss to get a bit more ram on the servers, but for now I just make sure I'm staying away from swap memory and everything runs smoothly.
(I'm running 5 mongrels on 1gig ram)