When upgrading a vehicles car stereo there are a couple of factors to consider. The first is which factory components can be integrated to work with the new receiver. While older cars have no issue retaining almost all factory components (speakers, antennas, steering wheel controls) certain makes and models make it more complex and therefore more expensive to retain and add certain features.
The second thing to consider is what accessories and additional components must be added to the equation. Almost all installations will require a dash kit or molded plastic to accommodate the new aftermarket radio into the factory location. Wiring harnesses, smart harnesses with embedded CANBUS decoders, and steering wheel control interfaces might be needed as well to allow the new radio to communicate with the factory system.
The last thing to consider is the disassembly of the vehicle and what panels/components must be removed to accomplish the installation. On some vehicles it may require just two screws on others it may require half of the vehicles interior to be removed to replace the radio. All these considerations affect the price and duration of an installation.