I'm not sure you would save quite that much...my mare was blanketed last winter, and she still ate the same amount of hay...I tried dropping her back a bit, but she would lose a bit of conditioning, so she wound up getting the same amount.
Cold is cold, and when it's that cold, they are going to have to eat to maintain their body heat and function, regardless of a blanket. I grew up in MN, and we never blanketed our horses, and it got COLD!!! They may have eaten a bit more hay, but I'm not sure it was 'that' much more, than if we had tossed a blanket on them. I would rather feed a bit more hay, than have to go through the hastle of blanketing all the horses; you have to take them off, and on, and make sure they aren't rubbing, etc...Maybe blanket some of them and see for yourself if it makes a difference that you find worth going through the extra effort of blanketing. The only reason my mare was blanketed last year, was because she was body clipped, and she may get body clipped again this year, depending on how well she does working in the winter (she would get really overheated and very sweaty last year, and that was just during our 'warm up' time! So I clipped her, and that made a huge improvement there).
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."