Actually, GST is paid on MHDs that have domestic internal travel. As to your original question, is the GST more than paying US customs/immigration and other fees which would increase the second ticket by about C$60. And you pay the same C$60 on the ticket that gets you to the US.
I thought GST was only charged on an international ticket when there was domestic connections. I have never paid GST on direct, nonstop flights to Europe.