From Toronto I would say that anything around $700 return per person taxes in would be amazing! If you read other threads, you'll notice that airfares seem to be higher this year as compared to previous years.
If you're interested in flying from Toronto, then it must mean you are relatively close to the US borders. I've flown down to SXM twice over the past two years, once out of Detroit through Charlotte for $399 return tax in and more recently out of Buffalo through JFK for $530 tax in return. The direct flight from YYZ is great but it will come at a premium as it originates from the greatest taxed land in the world!