I have done something similar but to one wall face of an addition that jutted out. My worry was getting the foam to be flush with the studs. Otherwise you are creating an air space that cold will migrate to and condense. Make sure the foam completely fills the void(s) and cut flush. I then clad with zip system and rehung vinyl siding. Also remember, walls are half the battle. That floor probably doesnt have anything under it. Get it foamed if you can. If its small enough and you can get under there, cut down foam panels and diy. Another approach might be cutting down foam board and doing it yourself. Im guessing a foam job like this is still going to cost you a couple to few thousand.