My best winter** cycling purchase so far has been a pair of fleece insoles I got from Mark's Work Warehouse for $4. They're thick white fleece with a foil backing. They've made a huge difference in reducing heat loss through the bottoms of my shoes/boots, and my toes have been much happier since I got them. I went in specifically looking for insoles; if I hadn't been looking I don't think I would have found them - they were right at the back of the store, on a little display stand facing the back wall.
**Now if we could just get some real winter weather. High of 6°C in Ottawa yesterday and today. In January. They're predicting rain tomorrow. This sucks.