Hello 'haslemere10',
<<Why has mailing a small magazine fhe US to Canada become sooooo expensive?>>
Because the GSP is good for Americans. 
What, that wasn't a riddle? Oh, okay, well GSP items are invariably more expensive than they need to be so the best thing you can do for yourself is find another seller.
When scrolling through the listings you are searching, these are the ones that have in grey,
Customs services and international tracking provided
That's a good clue to avoid it entirely and not even bother to open it up no matter how good it looks on the outside. A lot of things in life are like that.
If you do make the mistake of clicking on the listing, they also shout out, "No additional import charges at delivery!"
Anything coming through the mail is expensive, and if you want to see for yourself how pricey items can be even for the cheaper First Class rates, see here:
http://ircalc.usps.com/?country=10054
Magazines can be heavy, and the more expensive publications tend to use that heavy costly paper. If you feel the shipping fee is disproportionate to the size of the item, you can always ask the seller what the packed weight is. The honest ones will tell you.
Otherwise just keep checking in off and on, -- sooner or later a more affordable listing will appear. That's one of the great beauties of ebay, - everything and anything that can be legally sold on the site will eventally turn up. Common items like magazines, - loads of those will come and go. If you have a specific one in mind, you can set up a Saved Search (or rather, "followed") and get an automatic email every time one is newly listed.
Happy shopping
