You can get a fairly accurate dealer invoice number by getting a Car Cost Canada membership. It's $25 or $30 for something like 10 quotes (each trim is a quote). For something as inexpensive as an Accent, the dealer profit margin is likely less than $1,000, though...
As for GL vs GLS, with the GLS, you get bluetooth, heated seats, fog lights, 16" alloys (instead of 14" steel wheels, what?!) and sunroof. Those are all desirable features, IMO. For an extra $1,800, that seems like a good price.