My cheapo discovery this winter was switching to woollen socks. Kind of the same thickness as cotton sport socks. CHF8- from Migros clothing section. Made a huge difference cycling in 2C to 5C weather. Could actually feel my toes after an hours ride Wool is great in that it still warms even if it gets a little damp from sweat.

I use a light polar fleece beanie/mutze with ear flaps under my helmet and gloves with "windstopper" material are excellent.