i have a wintery headband for the ears for temps between about 20 and 40. Below 20 I use the balaclava

Gloves are also a big thing. I can get by with a windproof cover plus some generic insulating gloves. Length of commute matters here. The longer you are out, the colder the fingers will get (depending on your gloves). But what I have noticed is that the biggest help is windproofing.

1+ on the long fenders. Go for a $30 price range Planet Bike model.

I think the studded tires are great once the snow falls. Nokian Hakkepelitta are the way to go. I bought some cheaper (half the price) Innova Tundra Wolfs last winter and about half-way through I regretted it (studs wear out fast).

Get lights for the dark. Planet Bike Super Flash tail light.

Regarding cost... just remember: If you keep biking through winter, compare the cost of bike accessories to a car payment and gas. The stuff I mentioned "only" adds up to about $220, with tires being almost half that cost.