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Great question. I've built two computers in the past, and am typing on one of them right now. It's been about 5 years or so and this one is still going strong. I've done a lot of research into this and have come up with a pretty solid conclusion that it IS an economical choice. I built this PC from the ground up. Buying components for the build is what makes the most difference. The components themselves are pretty affordable, even the mid to high tiered components. I put probably $1500 into the whole build, operating system included, and I would say if I bought a computer from a store with the same specs, I would have spent double more than likely.
The great thing about building your own is that you can save money by adding the parts that you want, and not having to worry about paying extra for things you don't want or need. PC component technology advances at a rapid speed, and the prices of components drop significantly after the latest and greatest is released. Think of it how Apple phones are. The new one comes out and the older models drop in price, but extrapolate that and speed it up x4, and you have the computer components marked. So you can build a pretty beefy PC for a very affordable price. I would say, you could build one that runs circles around mine for an easy grand.
One last thing. My philosophy behind building instead of buying is not only because it's cheaper, but because I believe computer manufacturers build their machines to fail eventually so that customers have to buy new ones in 5 years. My first PC ran for 10 years until the motherboard became obsolete. The one I have now still takes everything that I throw at it and then some even 5 years later.