Stereo systems and stereo headunits are the most common aftermarket car upgrade, likely because it can be one of the cheapest. While headunits and stereo equipment can be expensive there are plenty of cheap and affordable options. For example speakers can be replaced for around $50 and headunits, of decent quality, can be found for under $100.
Cheap car audio is a popular upgrade for several reasons. First off even cheap audio speakers can produce great sound and quality base. Cheap or moderate range subwoofers and amplifiers can create the clarity and bass that is so important in car audios and new headunits can quickly and cheaply upgrade the entire stereo system to accept newer music formats and input devices.
Speakers, first off, are the circulatory system of an audio system. They are the devices that pump the sound out. While there are speakers that can cost hundreds of dollars cheap speaker typically work well as long as they installed properly and only true audiophiles will be able to tell the difference between high-end speakers and moderate speakers.
The headunit, secondly, is like the heart and soul of the entire sound system. Cheap headunits can be found on most websites and offer consumers the option of upgrading or replacing a failing or subpar component with something new. While headunits with added features such as bluetooth connectivity and video screens are extremely expensive a basic MP3 player headunit with auxiliary inputs (for iPods and other MP3 devices) can be found for under or around $200. Even well known audio companies like Sony and Kenwood offer entry-level headunits.