I think that you will have a real tough time trying to find a cruise for thbose weeks from either New York or Philly for under a hundred dollars/day. The cruises from the Northeast usually sell out pretty quickly, and the prices stay pretty high. The cheapest cabins usually seem to go for between $150-200/day.
That said, it doesn't hurt to call a travel agent to see what they can actually do for you. I'm not a TA, and I'm just going by what I see in the adds and from some previous wxperience. We live in Philly, and my wife doesn't really like to fly, so we have sailed from NY several times. The one good thing is that you can save the air fare. Good luck.
Howard