I travel often from NYC to Montreal. Flights are almost $400. My cheaper alternative that I do frequently is to fly into Burlington ($49 on USAir or JetBlue) and take the Greyhound/Vermont Transit bus to Montreal. No other way that I know of to get from Burlington to Mtl.
It is actually not bad at all. It's only a short $12.00 cab ride from the Burlington airport to the Greyhound bus station downtown. The cheaper option is to take the Burlington city bus (I think it's $1.25) which stops every 30 minutes at the airport and you are in downtown Burlington in about 20 minutes. The ride from Burlington to Montreal is about 2 hours... beautiful New England scenery.