Soldato
For Canada and the US I'd be impressed if you found flights under 400 notes. Given that my entire weeks accomodation + flights + transfers + food to France is going to cost £430 it wouldn't really make sense looking across the pond on the basis of exchange rates.
You'd be surprised, I've seen deals to the US and Canada for less than £400 (self catering).
Of course the deals are generally specific dates and locations that not everyone wants, but if you're not that fussed there are deals to be had.