Proportionally, the number of available M3s in Canada is close to the US (ie Canada has 1/10th US population, also about 1/10th M3s) so that isn't an issue.
Typically, sports cars in Canada cost more than in the US after conversion but virtually all other cars are cheaper in Canada.
The main reason is the Cdn M3 new costs much more than the US M3 new. Right now, exchange is about $1 US = $1.30 Cdn but new Cdn M3 price is set as if the rate were $1 US = $1.60 Cdn. Used prices heavily account for what new prices were, so while the depreciation rate for Cdn & US M3s are similar, our used M3s cost more because the new price is also more.