Not sure if this option interests you, but Halifax-Montreal-New York (with VIA and Amtrak) comes out to about US$330 round-trip booked well in advance.

Travel time is about 20 hours on the way to New York (though you'd chance a misconnect if the VIA train arrived in Montreal late) and 56 hours on the way back to Halifax (forced overnight in Montreal--Amtrak arrives after VIA departs)/

Sounds long, but remember, train travel in North America is fun and relaxing and can be a vacation in and of itself (much like a cruise)!

In any regard, $550, $425 or even $330 sounds ridiculously expensive when Delta had an ANC-YYG fare for under $400 filed for most of 2007 (I took it in August and drove to Halifax for a day). I thought I saw ANC-YHZ for under $500 sometime in 2008, too. Sorry I can't help further...