Thirty bucks isn't much of a drop, honestly. Could have just been a warmer month anyway. A little draft never killed anyone, it's just some moving air. My windows have whistled for years and it's fine. You sure it was that serious?
Last winter, a friend of mine ignored a draft like that. His heating bill shot up by almost fifty dollars a month. He thought it was just the weather getting colder. Then he found cold air pouring in around the window frame. A tube of caulk and an hour later, the draft was gone. His next bill was back to normal, so that thirty bucks sounds right to me.
How bad was my old caulk job? I used so much it looked like the window was sweating. Blair_hill59's friend is smart to fix it fast, my place used to sound like a harmonica band in a wind tunnel.