"...fire retardant spray can foam (behind the electrical boxes) could save almost that much per month."
I've looked into this, spray foams such as Great Stuff Fireblock are not UL rated for this use. With enough heat or direct flame, they do burn. I tried this with a lighter, I have lit this stuff and it does not immediately extinguish when the lighter(not blow torch) was removed. Fire blocking spray foams are fire blocking in the sense that they prevent a cavity from having access to air, which will suffocate most fires. Electrical fires are different than this, they continue to produce heat in the absence of a fuel, this makes such a product as "Fireblock foam" unsuitable for this application and the product is likely to burn.
UL1479 fire rated sealant exists for this purpose as specifically labelled caulking products.
Here is an example of such a product:
3M 10.1 fl. oz. Fire-Barrier Sealant Caulk-CP-25WB+ - The Home Depot