How to solve Hot Upstairs but cold basement
George Anderson is right. Balancing air flow is the easiest and most cost effective solution. Obviously, cold air is heavier than warm air. In the summer you need to direct AC air flow to higher floors and restrict it on lower floors. In the winter you reverse this directing heat to the downstairs and letting it rise via convection to upper floors.
Ceiling fans also help if used properly. To cool use mid to high speeds blowing down in the summer. In winter use slow speeds blowing up in order to bring warm sir from the ceiling down the walls.