I went through this exercise 3 weeks ago. I found the earliest Vancouver to Toronto room fare available. I then compared Seattle to Detroit. It was 40% less.
I did it just now. All the figures I used I got today, using 11/3/2023 as a departure date. You can check at viarail.ca and Amtrak.com if you doubt them.
Detroit is not equivalent in either trip duration or accommodation. New York is a closer approximation in both trip duration and accommodation.
Finally, this is an analysis that is neither theoretical to me nor new. I take a Seattle to New York trip every year in November, always by rail. Usually VIA one way and Amtrak the other. I track VIA and Amtrak prices for it and have for years, especially because I won't pay high bucket on the Amtrak side. 40% higher for VIA was generally in the ballpark, though a bit on the high side, years ago, before Amtrak became so aggressive in yield management and exchange rates were not quite so favorable. Today, 40% more for
VIA is an outlier that reflects a lowest bucket Amtrak fare that often cannot be obtained. The 25% lower fare for VIA that I found today may also be something of an outlier, reflecting high(er? I didn't check my fare results against the chart) Amtrak buckets. But, as many, many posts in this thread reflect, high(er) buckets are much more commonly available than low bucket is. It helps a bit that at least VIA's fares aren't volatile like Amtrak's are, so at least one side of the equation is remains stable.
Bottom line, on any given day, at least in VIA's off-peak season, chances are VIA's fares will not be significantly higher than Amtrak's, may often be lower, and may very occasionally be significantly higher, all depending on where Amtrak's volatile fares are for any given departure.
And VIA is a much better experience than Amtrak generally is. I've done both, a lot.