Nordstrom actually uses a service called Borderfree which operates in a manner similar to the GSP.
For grins and giggles, I compared listings for a kitchen appliance (Nostalgia Electrics breakfast maker) on eBay to the listing on the Nordstrom website.
Nordstrom wants C$96.79 for the item, C$30.39 in shipping, and C$27.44 in import charges, for a grand total of C$154.62.
An eBay listing with direct shipping to Canada has an asking price of about C$124.63 and ships by Priority International Mail for about C$114.86, for a grand total of roughly C$239.49.
An eBay listing using the GSP has an asking price of about C$113.05, a shipping price of about C$17.83, and import charges to BC of about C$15.46, for a grand total of C$146.34.
In the case of this item, at least, I'd be musing about how expensive it is to ship by mail, not by the GSP.
As with anything sold on eBay, though, the most appropriate and least expensive shipping method will vary according to the nature of the item. You may be comparing smaller items, which tend not to ship very economically through the GSP. Or you may be comparing items where the eBay seller hasn't chosen "free" or inexpensive shipping within the United States, thus bumping up the shipping rate to Canada considerably.
For me in BC, at least, I don't see anything out of line with the shipping and import charges levied by the GSP compared to those levied by Borderfree/Nordstrom. Your mileage may vary depending on the nature of the item and how taxes of GSP items are handled in your province or territory.
In any event, I think it's more useful to compare the total overall cost of an item (because, after all, that's what you'll be paying) rather than focus on any one component of that cost.
