There is film insulation that comes in large sections that are pre-cut.
There is shrink fit window film insulation, caulking, weatherstripping, spray foam...all inexpensive
For reducing exterior heat there are window films that will prevent solar gain through windows. These can be bought from home improvement and hardware stores. Most are static cling and go up with water. They are permanent, but more easily removed than those using a glue.
For reducing cold, start by weatherstripping and caulking to reduce air infiltration. The warmth inside your house is constantly lost through the windows. New double or triple-pane windows have small spaces between the layers of glass that hinder the loss of heat. Storm windows may be expensive but provide a reusable method of both stopping air infiltration and providing airspace between the outside and the window glass. There are also companies that build interior window inserts that work like storm windows. Look up "window inserts." The shrink-fit window films work by providing an airspace between the glass and your interior space for a much smaller cost.
The oldest method of reducing cold/heat infiltration is drapery. Translucent shades will allow some light in during the day while diminishing heat transfer through the window glass. Closing drapes over all the windows during night will best keep heat in your home. The heavier and tighter the drapery fabric the better insulation it will give.