Let's walk through building a similarly speced Hackintosh and set aside the build quality and all-in-one arguments for the moment.
(Massively cribbed from TonyMacx86.
Let's get as 3.2 GHz i5 for $200 (Core i5-4570).
We need a motherboard to plug it into. A Gigabyte for $142 will get us WiFi and some nice features (GA-Z87N).
8 gigs of RAM for $85 seems reasonable and compares to the target too. (CMZ8GX3M2A1600C9)
A Bitfenix Prodigy is a nice case for $90. Here, you may be able to go cheaper, but you can certainly go more expensive. (BFC-PRO-300-WWXKW-RP)
A Corsair 500W power supply for $55 is pretty reasonable.
Although I'd prefer an SSD, we're comparing to a system with a 7200 RPM spinner. A Seagate Barracuda for $79 seems appropriate. (ST31000524AS)
I'm having trouble matching the GeForce 755M with my Wikipedia-Fu. A modest video card with twice the memory sets us back $100 (ASUS GT640). Hopefully the performance is similar, but I'm open to suggestions.
The barebones system is $751.
You can get a 27" IPS display from Monoprice, which Anandtech said badly needed calibration to be taken seriously (http://www.anandtech.com/show/7240/monoprice-zerog-slim-27-ips-monitor-review) for $390.
You can get a decent keyboard for $50, and a decent mouse for $50 (here, you can beat both by downgrading, but I use a trackball that's closer to $100).
The reference system excludes an optical drive, so we won't needlessly add one to compare, but includes an SDXC slot, whatever that is. Assembling all those spare parts above gets me to $1241, and excludes software (which is famously "free" now, but really is only free with the purchase of a licensed computer), but I can save $460 over the reference system.
I cheaped out on the screen, but for another $100, I could get a Dell that's got decent factor calibration. I don't have speakers -- $50 may be a good budget for what's in the iMac, I don't have a camera, but a Logitec C920 for $75 seems equivalent. Adding those back in gets me closer to $235 under the reference system.
My hand-built system isn't an all-in-one, which is a value to some. My hand-built system may not be as quiet, which is worth a premium too (I used higher-power desktop components instead of the laptop equivalents in the iMac), and I may use more electricity, increasing the TCO by as much as $0.05-$0.10 per day (wild guess) which adds up over a few years. All of this, the OS, iLife and iWork licenses plus the support of being able to walk into an Apple store is where the $235 goes toward. My iMac is 5.5 years old. I've replaced the hard drive 4 times (one died out of warranty, the replacement was slower than hell but free, replaced that with a faster spinner, replaced that with an SSD), and the number of Torx screws necessary to get to them is significant, but does not make it unserviceable. The memory in my wife's (same age) died at 5 years old, and that was a $40 replacement that took 5 minutes.
It is unfair to say that this is a $12 burger selling for $100 (when I go to the local restaraunt, I pay $9-$10 for a burger, and it comes with fries... Are you overpaying for your hamburger?).