Retrofit double glazing 

Instead of replacing windows, only glass panes are replaced with double glazing panels into the existing window frame. There are companies that specialize in this type of installation. The glass used is usually very high performance which offers very good end result in timber frames. Aluminium frames reduce overall performance of the window. It will conduct the heat into the house in summer and remain cold in winter increasing the risk of condensation. Cheaper than double glazing but not by much

Secondary glazing

Involves adding a new pane of glass in front of single glazed windows. Glass or acrylic. When professionally made it comes framed attaching to existing windows by screws or magnets. Professionally made secondary glazing cost is around 1/3 of double glazing, and offers good performance overall. Can also be done as DIY project by cutting clear perspex to size and attaching to windows (with spacers if necessary) in which case the cost will be only a fraction of double glazing

Heavy drapes with pelmets 

Tight fitting heavy drapes with pelmets are said to work better than double glazing. The downside is that the drapes have to be closed to work which will also stop the light. So it is a good solution at night or in bedrooms. The cost will depend on existing window covering. 

Window film

Window film is attached to the inside of the glass. The film reflects the heat and slows down heat transfer. The cost is around 1/4 of double glazing, the performance is similar, sometimes better. A lot of different films are available, it is important to get the right one. In mild and cold climates "low-e" should not be used as it will reduce heat gain in winter, requiring more heating. To maximize performance in these climates (ie in Victoria) the film should be low-u (not low-e) and high heat gain, combined with good external shading.  

Corflute 

Corflute looks similar to cardboard, except it's made of plastic, clear, white or black. Clear plastic will let some light in but it will no good visibility. Good for secondary glazing DIY project, easier to cut than perspex and cheaper. 

Bubble wrap

This is truly budget solution, but we read a lot of good reviews form people who used it. Bubble wrap is cut to size, then applied to the window slightly wet with mist of water, bubbles to the glass. Can be removed without leaving marks and the bubbles and space between them create double glazing effect. Cost next to nothing, lets light in but visibility is blurred.  

Shrinkwrap film

Clear film cut to size and attached to window frame with clear double sided sticky tape then shrank to remove wrinkles. When applied correctly it's invisible and does not obstruct the light or visibility. Can be applied to individual panes to allow opening of windows or to the window frame for better performance. Has good reviews and we are currently practicing the installation to offer it as cheap alternative to double glazing. Downside - cannot be removed and reapplied. Removing clear double sided tape might take some time to clean the glue residue.