For New Yorkers: What is a good amount to tip your Super, Porter, and part-time Doorman for the holidays?

I'm living in a mid-scale building in New York with a part-time doorman, a porter and a super. They are great guys and very helpful, they keep the building very nicely. I am an artist on a tight budget and I wish I could be more generous, but can't really afford it. Any ideas of what would be a good amount to give without being insulting?