Just for comparison sake,
http://www.costco.com/ZT-Desktop%2c-Intel%C2%AE-Core%E2%84%A2-i7-3770%2c-3.40GHz.product.100011275.html
Something like that would be, using newegg (not linking, but can if interested):
$280 for a sandy-bridge i7
$60 for an inexpensive LGA 1155 motherboard; will be similar quality at its worst, but likely better than the costco one
$60 for 16GB* of RAM (compared to 12GB; cheaper to just get it in increments of 8 and better for latency; it's a good assumption that the costco is running DDR3-1333 at best, compared to a higher clock-rate from newegg)
$10 for cheap wireless adapter
$70 for a 7200 RPM* 1TB hard drive (not many 2TB drives in a 5900 RPM speed, so I just went with a smaller capacity, faster drive. General use won't see a full TB anyway, but a slower speed will hinder boot time and program load time- smaller and faster>bigger and slower, at least in the world of hard drive performance..)
$70-100 for Windows 7/8 (this goes on sale constantly; 8 was $70 last weekend)
$15 for a DVD burner
$25-40 for a cheap case; Rosewill usually has some decent small towers with good build quality.
$25 for a basic, quality power supply; the one included with the costco one is absolutely a cheap one, and I would say a cheap power supply more than anything is the source of most hardware issues.
Roughly $615-640 depending on if you grab things on sale online.
So you're maybe $50-100 cheaper to build it yourself, though the parts are across the board better quality and faster when you buy them yourself (increase in RAM capacity and speed and increase in HDD speed noted), and I would shave a further $100 off by going with an i5 or even an i3, depending on your specific use. Buying the quality parts is what makes the difference too; buying bottom of the barrel parts could save you a good $100-150 more, but then you'll potentially deal with headaches which isn't worth it in my opinion, and I'm guessing that's what many of these parts are.
However.. this Dell from costco for only $100 more addresses both the RAM and hard drive issues I brought up, has a cheap but better graphics card, and comes with a full version of office home, which can be $100 for just retail alone. I would still prefer to build my own, but I would have a harder time and probably couldn't match the price on this one while it's on sale:
http://www.costco.com/Dell-XPS-8500-Desktop%2c-Intel%C2%AE-Core%E2%84%A2-i7-3770-3.4GHz.product.100011674.html