First let me start by defining what I mean by "computer", or it might be easier to tell you what I don't mean. When I say computer I am not talking about the keyboard, the mouse, the monitor, or the printer. I am talking about the box all of that stuff plugs into.
For a computer to be working it needs 8 things, or parts, to make up the whole.
CPU (or Central Processing Unit)
System Memory (or RAM)
Storage Memory (often called Drive Space)
Graphics Processor (to make the monitor work)
Case (to hold all of the above).
I'm going to go over each of these briefly.
CPU (Central Processing Unit)
The CPU is the brain of the computer. It does billions and billons of calculations every second to make your computer do what it does. The CPU is the biggest factor in how fast a computer performs. CPUs have a rated speed (in Gigahertz or BILLIONs of calculations per second). So a processor rated at 3.7 GHz would (nominally) be faster than one rated a 3.2 GHz. However modern processors have multiple "cores" and each core is doing the assigned number of calculations at the same time. So a processor with 6 cores running at 3.2 GHz ( 6 x 3.2 GHz = 19.2 billion calculations) could be faster than a 4 core processor running at a faster speed (4 cores x 3.7 GHz = 14.8 billion calculations). In todays market, CPU pricing can range from around $80.00 to over $600.00
The CPU is by far the component in a computer that generates the most heat, and heat is the enemy of a computers performance and lifespan. All modern CPUs will need to have some kind of cooler installed. Fortunately, most of the value to mid level CPUs come with a cooler in the box when you buy them. Higher performance processors however will need some kind of additional cooler installed. Cooler pricing ranges from Free (included with some coolers) to over $400.00
If the CPU is the brain of the computer, the Motherboard is the central nervous system. It connects the processor to every other part of the computer: video out, keyboards and mice, hard drives, and it distributes power taken from the power supply to every other component as it needs it. Motherboards must be matched to the type of processor used in the computer by both brand (Intel or AMD) and age of the processor. The choice of Motherboard directly effects how upgradable or expandable the computer will be over its lifetime.
Current Motherboard pricing can range from as little as $50 or $60 dollars for less powerful and older CPUs to over $1000 for the top of line Motherboards for Intel's newest 12th Generation CPUs with the newest and fastest type of memory.
System Memory (RAM)
As the CPU does its billions upon billions of calculations the initial value of those calculations and results of those calculations are held (or stored for a short time) in system memory. Having enough memory to handle all of this short term storage, and memory that responds to the CPU quickly, is probably the 2nd biggest factor making up the overall performance of the computer. Only the very lightest of computer tasks (simple web browsing, light e-mail applications) will perform well with a system that has only 8 Gigabytes of RAM. In todays computer world, any business class machine, content creation (videos or photo editing) , or PC Gaming should really have 16 Gigabytes of RAM as a starting point (and Video Editing machines often will START with 64 Gigabytes before deciding if the tasks being performed really need 128 Gigabytes).
As I write this, 16 Gigabytes of RAM can vary in price from around $55 all the way to nearly $600 for the newest, fastest RAM needed for some of Intel's new 12th Generation Processors.
Storage (Drive Space)
System Memory (or RAM) is Volatile; meaning that when the computer is shut down, everything in System Memory is lost. Long term storage of your data (files, photos, videos) is done on some king of storage drive. This use to be a device not unlike a record player that had a physical disk that spun and a "needle" or read head that moved back and forth to the place on the disk that had the data you wanted to access.
Today, however, storage is more often held in Solid State Drives. These devices have no moving parts, are smaller, quieter, and more robust physically than a spinning hard drive. They also read and write data anywhere from 5 times faster (standard Solid State Drives) to 35 times faster (for NVMe Solid State Drives). Moving from a physical hard drive to some kind of Solid State Drive is the easiest way to improve a computers Boot Speed (how long it takes to load Windows when you turn the computer on).
As far as pricing goes, a 1 Terabyte Spinning Hard Drive starts at around $35. A standard Solid State Drive around $75, and the NVMe Drive around $95.
Graphics Processor (Video Card)
Some part of the computer is responsible for making the video signal that makes the monitor (or built in screen) show a picture. Some CPU's will have this ability built in to them for basic video out that will suffice for many users. Users doing high end video work or users running todays PC Games will need need more than the video provided by the CPU and would need to add a separate Video Card to their machine.
Entry level cards (for users who's CPU does not have built in video) are currently priced around $75. High power cards for extreme PC Gaming or Video / Photo editing can run into $2000 - $3000 range.
As the name suggests, the Power Supply takes the electricity coming from your wall and converts it into the various voltages used by the computers components. Light duty computers without any extra graphics cards can generally get by with Power Supplies rated at 400 Watts. Adding high end CPU's and top of the line Graphics Cards (each requiring extra cooling) can push the power needs to 1000 watts or even higher.
Power Supplies are rated by the total power they can provide, as well as how efficiently they provide that power. A "Bronze" level Power Supply may require 700 watts of power from your home to provide 400 watts of power; while a "Gold" Power Supply might only require 500 watts of power to supply that same 400 watts (and costing you less in your monthly electric bill).
"Budget" or "Cheap" Power Supplies also do not provide power that is as "clean" as a more premium power supply. Cleaner power is better for you computer.
A "Bronze" level 450 watt Power Supply costs around $30 - $50 dollars, while a "Platinum" level 1200 watt Power Supply can cost as much at $600.
To put it simply, the case is just a box that all of the other components fit into. Computer cases are chosen by size, airflow (to help keep everything cool - and remember to give it room to breathe), number and type of connection points on the front (USB Connections, audio jacks), and then more esoteric things like color, and general design. The smallest cases (called Mini-ITX) cost more than mid-sized cases and require special Mini-ITX MotherBoards and Mini-ITX Power Supplies (and small here means more money). At this time a base mid-size no frills case costs around $40, and top of the line "enthusiast" cases can run over $400.