If you feel that your windows are due for upgrading but don't have the financial means to do a major overhaul, then you should look at some cheap window insulation options on the market.
Storm windows go right on top of your existing windows. Generally installation of interior storm windows is a breeze compared to that of the outdoor variation. Storm windows can be made of similar materials used in conventional windows, and they are aluminium, vinyl and wood. Due to its susceptibility to expansion and contraction throughout the seasons, wood storm windows need frequent maintenance. Aluminium windows are good heat conductors -- unfortunately not something you want in summer. With little maintenance requirement, vinyl probably makes the best choice as the cheap windows insulation.
Solar insulation film makes cheap window insulation by blocking out over 50% of the heat from the sun, and at the same time filter away all the ultraviolet rays (or UV). As an additional note: certain manufacturers insist that their film needs to be professionally installed. Try to avoid the metallic based films as these are known to cause corrosion in humid area or by the seaside. Additionally they sometimes cause interference with cellular or wireless signals. The alternative, clear non-metallic film is a better bet. It represents effective and cheap window insulation and can block up to two thirds of the sun heat coming through the window, so you don't have to be as dependent on an air-conditioning system and you are able to bring your utility bill down. Since these films are transparent in nature, sunlight is able to pass through the windows, and homeowners are able to cut down electrical lights use.
Caulking and Weather-stripping
While window upgrading is normally a top priority job when a sufficient budget is available, homeowners with smaller budgets can still work on reducing the air loss from their windows. Use the weekend to caulk and weather-strip your doors and windows (you can find caulk that is specifically made for windows). Physically preventing air loss can provide more instant gratification compared to upgrading the window glaze. A proper installation of caulk will be able to achieve cheap and effective window insulation.
“Smart Windows” are the latest fad happening in window design. Smart windows technology is capable of changing the settings of the window based on the influences on the external environment. This can be enabled either automatically (also known as photochromic and thermochromic smart windows) or using a switch (electrochromic). The working principle of photochromic windows is similar to those intelligent sunglasses; i.e. it gets darken when you are outdoor. These windows are sensitive to a change in light and would response accordingly. On the other hand, thermochromic windows automatically switch from clear to solid white in the presence of direct heat.