Common Road runs across the Wine Yard

Alex and David both have wine yard. During rainy season they can't access the fields due to waterlogging.

Since the fields are near by, they can build a road. It costs $5K per Km. If they build individually, Alex requires 1Km road and David needs 2 Km road. If they decide to build jointly, it will be 2Km.

How do they share the costs between them?