From my limited experience and knowledge, prices for SxS's in Canada are lower, but availability and selection are also much more limited. I will let canvasback and gunsaholic comment as they have a lot more knowledge in this area.
Yes, in the majority of cases one will find the values of vintage doubles in Canada is probably at least 25% lower than what may be realized in the US. That is for a gun of comparable options and condition. However, there are many sellers in Canada that do not understand or want to accept that fact They also don't realize it is a very small market here until they find they get zero interest in their listings when the price does not reflect typical selling prices for the market here. Same scenario applies to many other types of used guns as well. Quoting US prices out of the "blue book" most of the time means nothing when selling guns here. But of course there are times when a seller may get lucky if a buyer doesn't know the market as it only takes one buyer!
