Yeah it tough. I’m in a bi-level and had a similar situation.

I live in Manitoba. I would rather spend the money on improving my attic insulation or my windows. Heat loss from your exterior walls aren't going to be as bad as windows, doors and your roof. I also would have tried to figure out a way to install those walls without losing a foot of the perimeter of my basement - like hanging them from the joists and gapping the bottom, which is standard practice out here. So that when your house shifts it's not turning those walls into something structural.