Sun reflective window films are great to reduce solar heat gains and glare, namely in hot climates. They can block up to 75% or the solar heat in single-pane glass windows, and 30% of the visible light.
Metalized vs. tinted films
Just be aware with the type of window film.
Tinted films (to simply color the window glass) are inexpensive but ineffective at blocking solar heat.
Modern metalized plastic window films are a much better choice.
Prices vary a lot. They may depend on the quality of the film, but also on brand policies.
Expect online prices around $40-$50 for 38/48 inches by 15 feet (see Amazon.com for reviews: Window films) rolls, and a lot more in specialty stores, namely when involving professional installation.
Some manufacturers require their films to be installed by certified installers, while others are promoting DIY installation. The installation is moderately difficult, and requires an absolutely clean glass surface.
When not to buy window films
Spectrally selective window films can be a very effective and low-priced choice to reduce cooling bills in hot climates.
But even the best films are rather ineffective or without significant advantages in homes with moderate or good energy-efficient windows.
The pros and the cons
Plastic window films can provide hundreds of dollars in energy savings each year, in hot climates. Other advantages: films can also cut glare and reduce furniture fading and, in some cases, winter heat loss…
But pay attention to their quality (there isn't an official rating system for window films) and their possible side-effects: some of them reflect indoor light at night, and block out views, and some of them are not perfectly clear.
Most homeowner complaints about window films involve their installation. Even films designed for DIY installation are not easy to apply.
Professional installation can be an important factor for the success and performance of window films, but contractors may charge you more than the cost of the film.