It has been said, and I agree, that whether this cartridge was from the Northwest Rebellion or not would not likely effect it's value. In my experience, Canadian artifacts of any type or quality do not draw premium prices.
Being that this rebellion was not considered a significant event, not even by most Canadians, it is unlikely that anything from those battles would fetch a premium price of any sort.
However, as near as I can tell, this particular cartridge is rare, mostly because of the headstamp. And that might give it premium status.
However, I'm no expert on such things so I'm only guessing. And I still have no firm figure as to a fair market price for this Cartridge.