If it's been around for 6 months they will shave a lot more off.

Longer lasting thing shave longer depreciation schedules, things that need to be replaced all the time have shorter schedules. 

Prior to that, perhaps seek to insulate YOUR area something (spray foam?)

I have the Makita.This is Woodworking 201 applied to decks, but you can mod the Makita with a graphite-impregnated pad between the belt and the base plate and an OEM replacement steel base plate that you superglue a foam block on.

Go to a big box store or online and buy a tube or two of mouse sealing foam and the applicator gun. 

it's been on the market for 2 weeks and they've dropped the price by 10k. it's now right at average for the area. add in the roof and driveway issues, and i shaved off 10k more which should be more than enough to address both items in this area.

If you don't like the studs on its side use metal cleats nailed to stud & then shoot nails through the metal cleat into the wall.Because of moisture I would then only use solid foam board insulation as its below grade that low down so you could probably get away with 2inches.

I am debating between spray foam and regular fiberglass.

so $420k construction cost, but you may be able to shave a bit since you have 2 houses basically under one roof. 

But for $5mm or whatever investment, I would do that near me, versus afar.How much of a shave is the relative willing to give up on the price, to me is the deciding factor.