How does one value one's time, anyways?
You know, more people should ask themselves this question. Everyone's answer is a little different, but they all boil down to an analysis of opportunity cost. You only have so many hours in the day/month/year/life. There are lots of things to do, but you can't do them all at once, so while you are doing one thing, you are losing out on the opportunity to be doing something else. For me, I value my hobby time at about 3 times what I make at work. Build a computer or buy a computer? Well, I'll probably spend 6 hours finding and ordering parts for a computer above what I would just ordering one. Then there's the time to assemble, and the added risk of building your own. Given my level of disposable income, the amount of free time I have, and the enjoyment (or complete lack thereof) that I'd have in building my own system, I'd probably have to save $2k to make it worth my time.
When I was in college, and I had very little disposable income, saving even $100 was worth it, so I built my own systems.
As a side note, this is why I should (and do) pay a higher tax rate. The value of my last $1k of income is much lower now than when I was in college, but the value of stable and effective governance only grows as I get more wealth.