Everybody has to "throw in" on this one -- the thread is already 3 pages long.  I'm no exception.
There was a time during the late '80s and through the '90s that you'd pay around $3,000 for an OEM rig.  I think one time -- I dumped 5 Grand into one.  I remember my landlord and colleague who spent at least that much in 1984 for an "IBM-PC."  I started building my own systems around 1996, and was able to save a nice pile of change.  
But that's "changed."  Looked in the Times today at the daily Fry's ad.  You can buy a low-end PC for $400 -- occasionally even less!!  For an aging parent, that might be fine.  I don't MIND being Mom's "tech-support" for hardware and OS maintenance.  But it's a "personal computer," and it gets a little irritating to be called upstairs for assistance with every "install new updates" and AV pop-up window that requires just a little decision-making and common sense.  Thank GOD for "remote desktop" . . . .  
Maybe you CAN'T buy a high-end rig for what it costs you to build it yourself, but it can come pretty close.  
For me, it's a personal choice -- acknowledging that its more costly and uneconomic.  I CAN'T STAND all the "extended care" software, internet reminders and other crap that Dell puts on their systems.  I can't stand the proprietary hardware.  I can't stand the fact that they may build it to your spec, but always throw in a PSU that only supports those specs and those needs at that particular time.  It's NICE that they're making desktops that are compact and have a low footprint, it's NICE that they come ready-loaded with Win 7 and some other complimentary software.
But I just can't STAND those things!  I CAN'T STAND them!  So I build a new machine every time it looks to be advantageous, and pass on the cast-offs to the fam-damn-ily.  I couldn't POSSIBLY have stuck with a Pentium 4 since 2006, but my brother did -- and he's perfectly happy with it -- except that he bought himself a C2D laptop.  And he's happy to get my Q6600 system at end of this year.
Hold on . . . I hear someone calling:  "Nor-man!!  NOR-MANNN!!  Leave that girl alone, Nor-man!!  Your mother did something to her e-mail browser, and doesn't know how to change it back . . . . NORRRRR-MANNNNN!!!"  And . . . there goes the Bates Motel, with nobody to watch the office and man the cash-register . . . .