I don't know about Vancouver's high schools
All I can say is, host family's cost depends... on what they include, and location, cost varies. I don't know if the average is $800
For school fee, if you're going as an international student, it's gonna be costly, especially once you're going to university -_-" And, at least in Burnaby, you still pay for student activity fee, and all that misc. stuff they'll force you to pay