So I really gotta shop for ammo where you do. Pre scamdemic, minimum here was $50-75/box. Saw some plain Jane at Canadian Tire for $92/box.
Somewhat Irrelevant as 45-70 is one of the calibers that most benefits from reloading. From a load of unique you could hand a 8yr old to stomper 500gr loads pushing the bottom of 458wm.
Even so, the Marlin guide, and other 45-70 are not dangerous game stopper rifles. IMO, they kill just fine, just don't stop them in their tracks. The 12ga, you mention would be better.
My 45-70 is a ported 1895g. Can't imagine, more don't want to know, how loud a short, short barreled 45-70 would be. Probably a fun range toy, but those nearby would hate you. Two foot fireballs at dusk for 1000 Alex.
Across, on the mainland is one of the most covered places in Kanada, for blackies and cats. I'm still not sure I'd pick my guide gun first. Actually should sell it. Dons flame suit.