Yes, you can get a low end PC for $450, made of cheap parts that have been around for a while and that are so prevalent that prices have come way down. For a gaming or CAD PC, you’re looking at more than the $1500 the caller mentioned. You get what you pay for. And the PC doesn’t have to control a bunch of machinery and stand up to heat, cold, moisture, vibration, and run reliably off a somewhat unstable 12-volt power supply. For 10-30 years or more.
That aside, you’re partially right–if the car is old enough, you can get the PCM or ECU, or ‘computer’ at a junkyard for a huge savings. And it will probably be fine, as these devices are usually the most reliable part on the car. You don’t have to go to an auto parts store or dealer to get gouged on prices; this is common in every industry. And a dealer will almost always be more, while a refurbished part from Autozone or wherever will likely do the job. You will still quite likely have to have the dealer ‘flash’ the new computer with the correct programming for your specific vehicle, taking into account the engine and transmission type, options, gear ratio, etc.
A car ‘computer’ is more akin to an industrial computer that runs production equipment. Your car is a process-controlled system, much like a factory environment. The computer has inputs from engine sensors, driver controls, and other networked computers in the car’s ‘environment’ It has outputs that drive actuators like electric motors, solenoids, relays, lights, fuel injectors, your car’s ignition coils, etc. All much the same as a factory controller that will take inputs from a production line and output commands to equipment to keep everything in sync. The hardware of the computer itself may be very similar on all cars that a manufacturer produces in a range of years, but the development, programming, and debugging of the hardware and software for a vehicle takes as much of a coordinated effort as any big-budget Hollywood release of a movie or cutting-edge game.
Oh, and if Microsoft or your PC mfr. gets something wrong, there may be a ‘patch’ issued or a recall to replace the problem at the vendor’s expense, but if a bug is found with a car computer or software, beyond the cost to recall all the cars for a replacement or software upgrade, and the bad PR and hit to the car maker’s reputation in this insanely competitive arena, it can cause safety issues, crashes, stranded motorists, more costly damage to other car components, and major lawsuits. People are a lot less tolerant of car issues than PC issues that they would just shrug off.
So to make a long story short, a lot of the development costs are included in the price of your car and replacement parts, and the hardware itself is usually much higher quality than the laptop from Dell or wherever that you just got. More like a military-spec. device than a home PC. For which we should all be happy.